The royal guest room of Shining Armor's Crystal Castle shed a dull, gleaming light from the sun outside onto the sleepy pair.

Nestled in the middle of the enormous, comfy bed, Princess Twilight Sparkle cuddled up closer to her brother, nuzzling her nose into his soft chest fur, perfectly content in the moment. “I love you, BBBFF.”

“Love you too, Twiley.” He squeezed her a little tighter around the middle, brushing his hoof over her little round rump for good measure. He enjoyed his marriage, but nopony could make him feel the same way Twilight did. He lay there quietly for a while longer, then stroked her purple mane. “The way you feel when you're on top of me... it's incredible... amazing.”

“Not as amazing as you feel.” Twilight looked up at him with a wide, lazy smile, her eyes half-open.

Shining chuckled. “No, no. You're way more amazing than me. But I guess we'll never know. There's no way we can find out for sure.” He looked down at his little sister, and he could see the gears turning furiously in her head. He knew there was no way that would end well. “Twiley, I know what you're thinking, and there's no way it can work. Body swap spells are too unstable. They're dangerous, and you know that.”

Twilight shrugged her shoulders slightly, then nuzzled back down into her big brother's chest. She slowed her breathing and relaxed, sure that her charade would hide her mind's studious work on figuring out a way.

Shining looked down at her... her sexy, curvy sides slowly rising and falling, the pleasant bulge of her rump just underneath the blanket, the darker purple spots on her fur where his essence was still drying into her coat. He had no illusions about what she was thinking; he knew her too well. He needed to distract her before she came up with something absurdly dangerous to try. “Cadance will be back tomorrow.”

“Hmmn...” Her only response was to move her hoof closer to his crotch – her own attempt at a distraction.

But he was utterly satisfied by now – he'd already given her three loads, after all – and that distraction wasn't going to work. “When are we going to meet up again?”

Twilight gave up on her attempt to, well, change the subject. “Doesn't Cadance have a trip to Saddle Arabia coming up in a couple months? I can find some excuse to send Spike away, too.”

That would work, Shining thought. It was a shame they could only see each other so rarely, but it did keep things fresh in bed. The way they had jumped at each other the first moment they were alone... He shook himself from that line of thought. If he let his mind wander in that direction, his little distraction might grow and really change the subject. “So, that sounds like it should work. Your castle or mine?”

The moment Twilight heard the elaborate crystal chimes of her castle's doorbell, she rushed across the great hall and flung the door open. “Shining, you came!” She tackled him to the ground, right outside her front door.

“Easy, Sis.” He laughed. “Let's at least get inside first.”

“Oh, I'll get inside alright.” She winked, and with a purple flash teleported them both up to her bedroom at the very top.

Shining looked around the place. It was different than last time – the bed was new: bigger, simpler, and less gaudy than the one the castle had come with, and she seemed to have completely removed all the gemstone and gold ornamentation. “Huh... I like what you've done with the place.”

She had no mind to talk about interior design right now. Her mind was elsewhere. “I figured it out!”

“You figured out your bedroom?”

“No no no.” She giggled. “I figured out how to do the swap!”

He stared at her, his face scrunching up. “The swa...? Oh.” He'd hoped she would forget about that. He should have known better. “Uh, Twiley, I'm not sure I'm up for—”

“I started off with body swap spells, and you're right: those are just too unstable and dangerous to use. No matter how much I tried to stabilize them, the results were just too unpredictable. I did once get it to be stable, but then it turned out to be permanent.” Twilight sidled up next to him and whispered in his ear, “If you see a squirrel acting like a frog or a frog acting like a squirrel around here, just ignore them... and whatever you do, don't tell Fluttershy until I figure out a way to reverse it.”

Shining winced. “But you said you figured it out?”

“Oh yes. Once that didn't work out, I started working with teleport spells, trying to modify one so we could teleport into each other's bodies.”

Shining just stared at her. Now might be a good time to leave... but then she spun around in her enthusiasm, and he got a nice glimpse of her tight little rump and the marehood nestled in it. He felt a little twitch deep inside his crotch, and leaving didn't seem like such a good idea anymore.

“I only tried it out on fruit to begin with, but that alone was enough to convince me not to try it elsewhere. Things got messy.”

He cringed.

“But the idea of teleportation got me going on a tangent of portal spells, and that's where I had my eureka moment!” She jumped forward, wrapped her front hooves around his neck, and kissed him before he could even react. “With a portal spell modified by stream-crossing it with a medical stasis spell, I was able to swap body parts just fine!”

Her face was just inches away from his, her intense purple eyes staring into his all too enthusiastically. Shining could feel himself getting worked up in good ways and bad ways all at the same time. “That's nice...”

“Let's try it out!”

Shining took a deep breath, trying not to stare at the way his sister's ass waved back and forth slightly, her tail flagged high above it. “I love you, Sis, but I'm not sure I can do this. It's going to be really weird for me, and I'm not sure I'm comfortable with the idea of—”

“You'd do it for me, right BBBFF?” She stared up at him with the big, shimmery puppy eyes she knew he couldn't resist.

“Is it safe?” Questioning the safety of it might be his last way out. Maybe he could at least delay her until—

Twilight dropped back to the floor and started nudging him toward the huge bed. “Perfectly safe. Rainbow Dash and I tested it out last week, and everything went fine.”

Shining's eyes went wide. “Wait! You and Rainbow are...?”

“No, no. Nothing happened.” She waved him off with a hoof. “Strictly platonic, strictly for science.”

He stared at her with his look, the one that could see right through her.

“Well, maybe not strictly magic science. When we tested distance limitations and we were separated for a while, I might have also studied what it was like to clop when I had an entirely different vagina. How often does a mare get to compare the subjective experiences of two different anatomies? How could I pass up an opportunity like that?”

His mouth gaping open, he visualized his sister playing with and clopping her borrowed marehood. His own genitals were not unaffected by such thoughts.

“You would not believe how sensitive that mare's clitoris is! Honestly, I'm jealous.” She looked him up and down, then giggled. “You're enjoying that mental image, aren't you?” Her gaze turned sultry. “Maybe Rainbow and I should swap again, and while we're swapped, I'll bring her more interesting organs over to try them out on you... She's very tight inside, you know.”

Shining's mind was in full fantasy mode now, swimming with the possibilities. He could barely bring himself to respond, “Hm... yeah.”

He wasn't the only one fantasizing about it. Thoughts of how big he'd feel inside Rainbow's tiny pussy were distracting her from... the matter at hoof! “Okay, let's try it!”

“Yeah!” Shining couldn't wait to try out Rainbow Dash's anatomy on Twilight's body, but when he heard his sister's magic lock and ward the door with a snick and a hum, he suddenly realized what he had agreed to, falling down to a sitting position. “Uh, Twiley, I'm not sure about—”

Poof.

Shining felt weird. As the violet-tinged magical smoke dissipated, he looked down at himself. His front legs and chest were normal, but the back legs poking out from beneath him were smaller than normal... and they were lavender. He stared at them for a while, unbelieving, before he gathered the nerve to fully inspect the damage. Leaning a little farther back, he could see where his white coat blended into Twilight's lavender one halfway down his belly. A hair-thin glowing line separated the two halves. Below that was entirely Twilight's body... in every detail. Cringing against what he knew would be there, he ran a hoof down and found not a sheath and balls, but a couple tiny teats. Experimentally, he tried to move a back leg, and it twitched just like he'd intended. He could feel his own hoof on his new teats. Everything was connected and everything worked. Weird.

Twilight smirked at him. “How's it feel?”

He looked up at his sister. He could have predicted it, but he was still shocked to see his own hindquarters on her body... and stunned to see her cock – his cock? – already bulging out of its sheath and dropping down between her legs. He'd never seen it from this angle before.

“A little stunned, huh?” Twilight shrugged internally. It had taken Rainbow a long time to get used to it, too. Apparently, most ponies weren't as comfortable with magic-induced transformations as she was. She took the opportunity to sit back on her rump and inspect her new appendage... thoroughly.

Shining couldn't look away, as much as he might have wanted to. His sister was lovingly stroking her new cock, staring at it as it rose up and hardened.

So this is how it feels, Twilight thought. No wonder they enjoy this kind of stimulation so much. That brought to mind another form of stimulation... She looked down at the blunt, rounded tip of it, at the hole already leaking a bead of pre-cum, and she licked her lips.

Shining's eyes shot open even wider when he saw his sister lick the tip of her own cock, then bend down even farther to take it into her mouth. It was causing strange sensations in him... There was a strange intense warmth in his rump, and a strange sort of itch. It was starting to feel like he was sitting in a warm puddle.

Twilight went down as far as she could, but she wasn't flexible enough to take much of the cock. Still, she tried vigorously, enjoying the new sensation even more than she had enjoyed her previous stimulation.

When the feeling became unbearable, Shining stood up, craning his neck around to look under his tail. What he saw shouldn't have surprised him, but it did. Twilight's pert, familiar pussy glistened on his own – now more shapely and much purpler – rump. A well-remembered feeling of arousal flushed through him from looking at it, but it wasn't physically manifested in the usual way. Instead, his new pussy winked hard, thrusting its clit out and dribbling a trail of shiny clear honey onto his inner thigh.

The noises from Twilight became more and more strained. “A little help?” she asked.

Shining looked back at her. At the partially wet, achingly hard cock she was still stroking. “There is no way I am sucking my own stallionhood.” He turned away from her resolutely.

That only gave her a wonderful view of what was once her own nethers. She could feel her cock pulse with the view. Standing up, she approached her brother slowly, staring at his borrowed marehood as it winked back at her over and over, dribbling onto the plush carpeted floor. He might not have been into it, but her pussy knew what it wanted, even when it was attached to somepony else.

She lifted him with her magic, flinging him belly-up onto the bed.

Shining had meant to say 'Okay Twi, this is too much, we need to stop,' but all he managed to get out was “Okay, Twi—” before she pressed his own cock against his needy new pussy.

He couldn't help it. His new anatomy could feel what it craved, winking against the flattened head of the cock prodding it.

Twilight pressed forward, and the tip of her cock pushed her own little violet pussy inward, then popped inside. She watched, fascinated, as it stretched to accommodate her, as slick juices escaped around the edges to coat her shaft. Her pussy felt incredible! So warm and soft... and inviting. It pulsed again, pulling her a little farther in.

Shining gasped. “Twilight!”

She just smiled down at him and pressed forward again, sliding in, watching every ridge of her brother's cock sliding into her own pussy, such a familiar sight, but not from this angle... and now she was feeling what he felt.

His lavender back legs curled around her white rump and pulled her in. She couldn't help but grunt as she was completely swallowed, hilting herself into her brother.

Shining stared up at her with wide eyes. He could barely breathe. Goddess, he felt so full. He could feel every twitch of the warm cock buried inside his pussy, and it only made him want it more. “Oh Twiley... Twiley...” He panted, his body shuddering.

She pulled out a little and plunged back in, driving them both back into ecstasy. She didn't use the long, elegant strokes Shining usually did when he was fucking her. Instead, she rapidly pulled in and out, moving only a tiny fraction out before pressing herself back in as deeply as she could.

Letting out a surprisingly girlish moan, Shining reached up and wrapped his hooves around his sister, just above her wings. He was clutched against her as tightly as he could manage, gasping in a breath with each of her tiny, quick thrusts. He could feel it, feel her sheath pressing between his – or her? – ass cheeks, could feel the stretching of his borrowed pussy, and he could feel the tip of her member prodding him deep, deep inside, putting a wonderful pressure on him that seemed to grow and grow. She felt huge in there!

Twilight let the pleasure burn through her, let her borrowed body parts' primitive need drive her. Without any practice in holding back orgasm, there was no way she could last. She felt the head of her new cock flaring wide at the tip, stretching her own pussy out deep inside, pressing up against her own womb... and she burst.

She could feel her balls clenching, her new stallionhood throbbing, as thick strands splashed into... her own marehood! Suddenly realizing her mistake, she pulled out as quick as she could... which was obviously not fast enough.

Shining squealed as the wide flaring head of his own cock pulled roughly out, stretching his borrowed pussy achingly wide. It popped out with a sploosh of fluids that immediately soaked his tail, only to rub up against his teats and his belly as it spewed a few more thick lines of cum across his chest and neck.

He stared down at himself. Most of the cum had ended up on his top half – the real him. It wasn't that bad, he struggled to remind himself. It wasn't as if he'd never gotten his own cum on himself before. He masturbated plenty, and accidents happen, after all. He curled up as his pussy spasmed, still burning with need. He hadn't orgasmed.

Twilight panted as her brother released his grip and fell back onto the bed. She watched his hoof dart down in between his lavender legs, hesitate for just a moment, then plunge in, clopping furiously.

She admired the view for a moment, then decided to take action. “Come on, now, BBBFF. What kind of sister would I be if I didn't help you out?” With a few flaps of her wings, she rose up, spun around, and backed down on top of him.

Shining's eyes went wide. His vision was filled with an insane view of his own heavy balls and flaccid cock dangling in front of his face... but he quickly forgot about that when Twilight pushed his hoof out of the way and went to work tonging his clit. His eyes clenched shut. It was unbelievable, sending lightning bolts of jittering pleasure all through him – none of his sexual experiences as a male had prepared him for this!

Twilight had to admit it was a little weird, licking up cum from her own pussy that she'd put there with Shining's cock... but she couldn't deny that it was turning her on... or maybe it was just the panting breaths and moans from below her as Shining's body convulsed with her every lick.

When Shining opened his eyes again, he was again greeted with the sight of the cock and balls he used to call his own. Maybe it was the heady cocktail of erotic hormones flooding him from his new female half. Yes, he could blame it on those... because he couldn't deny the odd allure they had for him now. His head rose up a little, almost without him asking it to, bringing his nose close the the dangling organ.

So smooth and warm and soft, still thickly coated with mare juice... he could feel the heat radiating from it. And those big round balls hanging just above, the way they swung back and forth just a tiny bit as Twilight moved... he knew he would deny it afterward, but he also knew what he was about to do. His head rose farther, and he nuzzled his nose into them, feeling their weight and fullness as he rolled them around, and then giving the smooth skin a long, slow lick.

Twilight grunted, and her cock dropped down from its sheath a little more, quickly beginning to return to its full length.

The more he touched it, the less it shocked and repulsed him that he was doing it. The feeling of soft, supple skin over the meaty core of that shaft as he worked his way down grew and grew in his mind. There was no feeling like it, that special warmth that he could feel from it.

She could feel herself getting hard again. She needed to focus! Thankfully, she was familiar with her own pussy, and she knew exactly what to do. She began rubbing the soft underside of her hoof in quick, firm circles around that tiny, protruding clit. When she heard Shining's gasp and felt his tongue fall away from her member, she knew she was doing it right.

Twilight's hoof felt amazing! Shining couldn't do any more than lay there and shudder, his abdominal muscles pulsing rhythmically as he experienced his first female orgasm. It was nothing like when he was a male – this was on a whole different level. A tsunami of bliss exploded from his freely squirting pussy and rolled through him unstoppably, setting him on erotic fire from the inside out, sending sparks flying all across his brain.

Twilight rolled off of her brother, flopping down onto the bed next to him. She propped her head up and admired her good work, laying a wing across Shining's belly – that just happened to brush feathers delicately across the little nipples down there – so she could feel the convulsions happening deep inside his half-and-half body.

Finally, it stopped. Shining breathed deep, then turned his head to stare at Twilight. “Wow.”

She grinned.

He glanced across her body. “You're hard again.”

“It's no big deal,” she said, waving him off. “I feel pretty satisfied.”

Shining smiled at her, then rolled off the bed, doing his best imitation of a sultry, sexy strut – which didn't turn out all that bad, given the circumstances and his lack of practice – until he ended up facing away from her, toward the locked door. He flagged her purple tail high and winked her own pussy at her. “Do you want to be pretty satisfied, or totally satisfied?”

Twilight was on him in seconds, her front hooves grabbing onto the color-matching rump beneath them. Still a little unpracticed at this, she leaned back, standing upright, so she could see what she was doing and guide herself in with her magic.

His pussy was responding to the new stimulus already, even though he was still quivering from his recent orgasm. Being female had benefits.

Holding the cock she'd borrowed from her brother in a ring of magic, Twilight guided it up to her former entrance, laying her flaring tip against the bulging lips of that little violet pussy. She could see a little flash of pink as the head of her cock slid inside – it was winking at her, begging for more. She gave it more, gliding the already-slick cock in to Shining's orgasm-relaxed pussy in one smooth stroke. The thick cock slid easily into its familiar home. It hat been there dozens, hundreds, of times, and the two fit together perfectly... but this time was different, entirely different.

Shining gasped and pushed himself back into his sister's thrust. He was beyond his inhibitions now. He craved it, more than anything.

Twilight gave him long, hard thrusts, her cock sliding out almost all the way before plunging back in each time. She couldn't get enough of seeing its mottled white bulk slowly disappearing between those velvety lavender cheeks, sliding across swollen and stretched pussy lips, and then plunging deep inside with a squelch of wetness.

She could feel Shining's pussy getting tighter and tighter as he tensed it up along with his increasing arousal. Did he learn that from her, or did he just figure that out on his own? She'd have to ask him later, but now wasn't the time. Instead, she kept thrusting into his borrowed anatomy, marveling at her pussy's ability to swallow the enormous cock. Now that she knew what the approaching orgasm felt like, she could try to avoid it, to prolong things just long enough for...

Shining was out of his mind with erotic joy. Nothing mattered but that marvelous cock pistoning in and out of his pussy and the sensations it brought. He couldn't manage any thought beyond the constant yes, yes, yes! his new-found ladyparts sent screaming through him. With every thrust, the feeling grew even better. He could feel the tip flare wide deep inside him again, and he knew what was coming. There was no risk... the damage was already done, after all.

He barely had enough in him to cry “Keep it inside!” before his body exploded again into a mind blowing female orgasm. This time, though, the supple muscles inside his pussy had something to clench against, and it was a whole different world.

Twilight felt the pulsing deep inside first, a deep throb that was soon joined by a warm spray around the base of her cock.

As Shining continued to cum like only a mare could, his back arched involuntarily, tilting the entrance of his pussy upward slightly, getting it ready to catch what it instinctively knew must be coming.

It was the feeling of his clit pulsing against the bottom of her shaft, rubbing the lip of her sheath, that did it. Twilight blew her second load of the night, pumping it deep inside her own pussy relentlessly. There would be no pulling out this time.

Waves of hot, thick cum splashed deep inside Shining. He could feel every spurt, and he could feel a warm, full sensation growing that could only be the feeling of a womb filling to the brim. He came again, harder than ever, right on the heels of the previous orgasm, before the one before had even fully finished. Being a mare was amazing.

Twilight gasped and shuddered as her cock emptied its last few spurts into him, watching enthralled as the nubile little pussy beneath her pulsed and squirted again.

Both of them stood like that for a time, each trying to catch their breath and waiting for the last tremors of orgasm to rock through them. Every residual twitch of Twilight's cock triggered a gasp from Shining, and every remaining clutch from Shining's pussy made Twilight's eyes go wide for a moment.

Finally, Twilight's borrowed cock softened and slid out of Shining's borrowed pussy. A milky-white sticky trail connected the tip of it to Shining's soaked entrance for a moment before breaking off and dripping down against his back leg. Twilight slid off his back soon after.

“Woah...” Shining mumbled.

“Woah indeed,” Twilight agreed.

He looked down at himself, at the trail of thick honey his pussy was leaking onto the floor. “Um, can we...?”

“Sure.” Twilight's horn glowed, and with another poof, the two were switched back to their normal selves.

Glancing down at her nethers – now that they were back in their proper place – Shining winced at the cum still leaking from her entrance. “Is there going to be a problem with—?”

“Don't worry,” Twilight assured him. “I don't think I'm in heat right now, so there's nothing to worry about... probably.”

He hopped onto the bed. “Good, good.”

She knew that invitation for what it was, and she couldn't be happier. She hopped up with him and nestled her back into his belly, taking her favorite 'little spoon' cuddling position. “I love you, Shining.”

“Love you too, Sis.”

Twilight pulled his forehoof around her middle and pressed her butt up into his crotch, and the two shared a moment of contented silence.

“Hey, Twiley?”

“Mm-hm?”

“Can you teach Cadance how to do that spell?”