Lost in the debate about the utility of the spell was some very simple issues with the amazingly artistic thaumaturgy that Twilight had just created. First and foremost, there was more than one alicorn.

If either Spike or Twilight had taken some time to think about the nature of the spell, they might have realized that swapping Twilight and Luna would permit the instant and effortless teleportation she’d imagined… though it might have given the lunar princess an unexpected Ponyvillian vacation for the duration.

But when it came to all things alicorn, Twilight tended to focus on her mentor to the detriment of the other two mares, Luna and Cadence.

When she cast the swapping teleport spell, the energy rippled through Equus’s thaumic field, looking for another great source of magical energy, like an asteroid falling in toward a gravity well. In most circumstances, this would have been Celestia, located only a few hundred miles away from Ponyville… or her sister, located in the same castle… In this particular instance, Luina just so happened to be closer to Ponyville - the tower of the moon was located further south in the castle layout than the royal dais.

But, Twilight would not be appearing in Canterlot at all.

By random chance, Cadence and Shining Armour were only a hundred miles from Ponyville. The prince had presented his beautiful wife with a private, romantic zeppelin cruise for their anniversary a few months prior. It had taken some time to arrange their schedules and some cajoling to convince Sunburst to foalsit their daughter Flurry Heart for an entire week, but they’d finally managed to pull it off…

And now they were making up for lost time.

Making up hard.

So it was that the swap spell snaked outwards from Ponyville, spiralling in toward Princess Mi Amore Cadenza’s location as she enjoyed some very intimate time with her husband… Twilight’s brother… Shining Armour.

The energy tendril connected.

*POOF*

A huge cloud of smoke engulfed Shining Armor, filling up the luxury cabin and obscuring his vision. He coughed and sputtered, blinking to clear his vision.

“Wow, honey, you should have told me you were close…” He chuckled. He was balls deep in his beautiful wife… when the sudden cloud of magical vapor had appeared, engulfing him - obviously the result of some magical overflow.

“Ughh….wh-wha…”

‘That voice sounded strange…Oh f-fuck… she’s really clenching down on me…!’ He reached down and tapped the mare on her shoulder. “E-ease up hun… you’re gonna snip me off if you keep clenching like that…”

“Sh...shining?” A groggy, confused voice came from underneath him.

As the last of the fog cleared away, Shining’s mind raced to understand what it was seeing.‘P-purple fur… dark purple mane… purple eyes… wings… horn… a magenta streak… the unicorn under me is….!’

“T-T-TWILIGHT!?”

“SHINING!?” Her eyes shot open with alarm, she was staring right at him.

“WHAT THE BUCK IS GOING ON!?” Shining shouted, half whinnying in panic… right below, him… under him… around him… where his beautiful wife had been just a moment ago… was his little sister!

“AHHHHHHHHHHHHHHH!” Their two voices echoed through the cabin, the sibling screaming in terror at the sudden scenario they found themselves in. Shining couldn’t help but buck and jump, instinct causing him to try to shake his little sister off of… him… as quickly as possible.

“S-stop! STOP!” Twilight squeaked at him.

“What the buuuuuck!?” Shining whinnied again, just managing to avoid kicking and shaking again.

“Wh-why the buck are you… inside me!?” Twilight screeched, staring straight up at him, flicking her eyes downward with panic as she did so.

“I DON’T KNOW!” Shining yelped. He could feel Twilight’s smaller body pressed up against him, her belly pressed against his… but most of all his felt the burning, twisting, clenching tightness below. Not that Cadence didn’t have one of the most voluptuously velveteen vaginas to ever grace the planet - she was the Princess of Love after all. But still - that babymaker had already fired one round and, snug as it was, it didn’t hold a candle to his little sister’s pussy.

‘My little sister’s pussy!’ Why. Why did he know how tight his little sister’s pussy was!?

She grimaced, squinting her eyes and clenching her forelegs against his upper back, wincing. “O-oh f-fuck… I can… feel you!”

“WHY ARE YOU… HOW DID… WHAT!? What!? What.” Shining’s confusion finally began to overtake his shock and fear, and his voice began to reverberate less in his ears.

“I… o-Oh… I m-must have… mnn…” Twilight tried to explain, but she kept pausing, her breath hitching. Each time she winced Shining could feel her… her… clench around him even tighter. “...s-s-wappped… into the Princess’s b-body...mmnf! B-buck it’s st-stretching me…”

Shining heard Twilight whisper under his breath about how his cock was stretching out her marehood. He was horrified to feel a pulse of throbbing arousal at her accidental compliment race through him. He felt his stallionhood twitch inside her, felt it rub against her inner pink flesh. His sister’s flesh. ‘Buck why does it feel so good!?’

“O-oomph! St-stop t-witching!” Twilight snarled, closing her eyes to try to focus.

“No, you stop squeezing!”

“W-why were you… mmn…! Bucking PRINCESS CELESTIA!?” Twilight grunted, finishing her statement with a snarl and a glare.

“I WASN’T!” Shining growled back down at her. “Cadence and I were having a relaxing and intimate love-making session when all of a sudden there’s a poof of smoke and all of a sudden I’m balls deep in my…”

“...” Twilight stared up at him, her cheeks going redder and redder, face flushing to the color of a tomato.

“...s-sister…” Shining finished, the lump in his throat barely making it out, in a little croak.

“O-oh m-my G-goddess…” Twilight squeaked with embarrassment. “My BBBFF is… inside me.”

Shining felt the heat in his own face burning and swelling with shame. A sickly feeling was quickly replacing the shock that he’d felt in his gut - here below him was his younger sister. The mare he’d given horsey-back rides to… the mare that he’d taught how to fly a kite… the little filly he’d grown up with, side by side… Affectionate, yes… but as siblings. As brother and sister.

He still found it awkward to hug her.

And now he was… well… they were… fucking.

“I-I… must have gotten something wrong with my spell… I was supposed to swap with Celestia…”

“Oh.”

“Oh? OH!?” Twilight growled. “That’s all you have to say!?”

“Well excuse me, Twily… I haven’t exactly practiced my responses to when a family-member magically appears on my…” He stopped short of saying the word… but it didn’t change the clenching squeeze around his stallionhood - Gods he could feel the ripple of milking mareflesh ripple down his length!

“...s-sorry…”

“...”

“...”

“...wait… sorry for shouting, or sorry for clenching on my-”

“SORRY FOR SHOUTING! I CAN’T HELP THE OTHER THING!” Twilight groaned, throwing her head back.

“Ah.”

“Just… let’s just uh… disconnect and then we can talk about this…”

“OK, OK…” Shining sighed, hanging his head with shame and trying to bury the horrifying feelings of disgust and fear… and arousal… bubbling up in his stomach.

“...Shining…?”

“Yeah?”

“WHY CAN’T I GET OFF OF YOU!?” Twilight shouted, wiggling and struggling under him.

‘Holy b-buck! When she moves her hips like that… f-fuck!’ Shining armor bit his lip to clear the flood of pleasure from his mind and respond to his little sister. “P-probably because of all the binding straps…”

“Ah.”

“Yeah…”

“WHY ARE THERE BINDING STRAPS!?” Twilight kicked out uselessly from underneath him. An elaborate system of hoof and back straps held her belly tightly to his belly. Belly-riding was what Cadence had called it. A mare was essentially bound to the underside of her stallion, upside-down. It let the stallion walk around as per usual, albeit with a heavier weight to carry… all while still rutting his female, like some kind of obscene sex-flesh-toy.

“C-cadence l-likes to use it when her legs get tired from when she’s r-riding-”

“Ahhh!” Twilight groaned. “Too much information, too much information!”

Shining looked down at her with a skeptical and cocked eyebrow. “Twilight, right now my dick is literally in your…”

“Way too much information!” Twilight squeaked, shutting her eyes. “Okay, okay… I’ll just… I’ll undo the straps and… Shining… why… why am I wearing a magic-restrictor horn-ring?”

“Umm…”

“Why are you also wearing a magic-restrictor horn-ring!?” She yelped, eyes wide with panic.

“Th-thursdays is earth-pony style day where we both pretend like we don’t have horns, and Cadence gives me the earth-pony-size potion so th-”

“I don’t wanna know!” Twilight whined out, exasperated. “How do you get out of the straps then… o-oh…. F-fu...mmmf!”

Shining felt a flush of heat roil through his face as he watched his little sister squirm and moan under him. A rippling rhythmic series of contractions clenched at his cock, little obscene grasping squelches loud enough for both of them to clearly hear coming from down below. And he could swear that he felt her tight body cavity get even hotter. He was ashamed to feel his body naturally make his thick stallionhood pulse with excitement.

“D-did y-you j-just….?”

“NO!” Twilight bit off quickly, looking at her big brother with angry, sparkling eyes. “J-just a mini… ahem… how do you get out of these?” She strained against the bindings.

“Oh. They’re enchanted to release when you say the secret keyword.”

“Thank Celestia, say the word so we can get o-”

“I don’t know the keyword.”

“Oh.”

“Yeah.”

“...”

“WHY DON’T YOU KNOW THE KEYWORD!?” Twilight screeched, throwing her head back (in this case down toward the floor) with fury.

“They’re Cadence’s bindings! I left my bindings at home!”

“Why do you have two…. You know what, never mind. Are you saying we’re trapped like… this?” Twilight gestured down their bodies toward their… union… with her eyes.

“I… think so…” Shining sighed.

“Oh… oh no…” Twilight despaired. Then her eyes blinked again and she looked up at her brother. “W-wait! You said… you said you took an enlargement potion, right?”

Shining swirled his tongue inside his mouth, tasting the last remains of the strange banana-flavored Zebrican brew. It probably wasn’t the best time to mention to his little sister that he had lapped the mixture right out of Cadence’s… cleft. Not only was it a sexier, but it often helped with penetration if Cadence’s cavern was a bit over-sized for earth-pony Thursday as well.

“Yep.”

“OK! Ok. So… all we have to do is wait for the potion to wear off, then we should get a little wiggle room and I can wriggle out of the bindings, or at least get off of your… uh…”

Shining arched an eyebrow as he looked down at his sister.

“Your penis.”

Shining swallowed. Why did hearing Twilight say penis sound so damn… hot!?

“Shining.”

“Y-yes?”

“Why did that make your penis throb? H-hey it just did it again!”

“Stop saying penis!”

“Penis is the correct term!” Twilight groaned as he throbbed in her again - damn treacherous dick! “Y-you did it again!”

“S-sorry… sorry… it’s just… when you say that it’s kind of… c-cute.”

“...”

“ANYWAY!” Shining blushed and stared away from his sister. “About your p-potion idea…”

“Y-yes?”

“See the thing is…” Shining gulped, guiltily looking back at Twilight’s face, meeting her accusative eyes. “I… uh… I had taken it just before you showed up… so I haven’t really… enlarged yet.”

He could see the color drain out of Twilight’s face, her mouth dropping open. “W-what.”

“Yeah.”

“You mean… th-this…!” Twilight clenched around him, unnecessarily providing a physical stimulous to remind him what she was referring to. “Is your normal size!?”

“S-sorry…” Shining meekly offered, ears folded back. He couldn’t think of any time that he’d apologized for being too big. Then again, Cadence was his first and only lover.

“H-how!?” Twilight moaned mouth parting with exasperation. “H-how d-do you have s-such a b-big… cock!? MmmF!!” She squeaked out as he throbbed inside her.

“H-hey!”

“Ixnay on the ockcay!” Shining begged.

“That’s what it is! It feels like its in my damn stomach! It must be…” Twilight let her statement trail off.

“It’s about seventeen inches long, maybe nine inches around.”

“Y-you measured!?” Twilight asked with a look of disbelief.

“I thought you liked science!”

“Seventeen and nine!?” Twilight blinked. “That’s l-like…one point nine times bigger then Flash Sentry.”

“Wait…. You measured!?”

“I like science!” Twilight groaned.

Ever shout was making her body rub against his. He could feel his little sister’s tight, hot, clenching marehood starting to drip down his shaft… could feel the press of a fat love-nub against the top of his ebony shaft. Both siblings shuddered together, the shiver rippling through them and making their fur stand on end. For a moment, all they could do was gasp and pant, staring at each other.

Then the moment was gone.

“W-wait… Flash Sentry!? That little punk from the royal guard!?”

Twilight rolled her eyes at her over-protective older brother. “It was just a short summer-fling and he’s not that b-”

“He dared to talk to you, go-out with you… touch you - without asking me first!?”

“See, this is why I never had any boyfriends through school! If you didn’t go so crazy all the time, maybe I wouldn’t have had to lose my v-card to Flash when I’m halfway to thirty!”

“He took your virginity!?” Shining felt his temper rising. His nostrils flared as he imagined a stallion mounting his little sister! He stomped his hoof and flexed instinctively throbbing his rigid length taut deep inside Twilight cunt. “He’s dead.”

“Mmmph!” Twilight squeaked, momentarily distracted before looking at her bother with a grin. “Sheesh, Shiney… it’s not like you need to be jealous or anything.”

“Jealous?” Shining looked down at her with confusion.

Twilight’s eyes flashed downwards and back up at him again, and her blush bloomed two shades brighter.

“Ah.”

“...”

“...”

“I mean… Flash was sweet and all… but… wow... “ Twilight mouthed with an O, wriggling her body around him. “Are other stallions like this?”

“Other stallions? ...Twilight…” Shining growled down at his little sister.

“Seriously… How does Cadence handle you?” Twilight asked with genuine curiosity.

“Twilight…” Shining could feel his heart beating faster as his cute little sister went on about his endowment… an endowment she was currently half-impaled upon.

“No wonder Flurry came out so fast.” She mused with excitement, as though she’d just figured out a puzzle. “Seriously...not even Rarity’s toys are this… uh...“

“...”

“Sorry, kinda got carried away.”

“It’s OK.”

“...seriously, mom would be proud.”

“TWI.”

She giggled, mischievously. “Sorry, couldn’t help teasing you a little bit, big bro.” And just like that, she’d somehow managed to switch back to the adorable and beloved little sister that he had known all his life. For a moment at least. Before the drip of her mare-juices off the end of his balls reminded him of the situation they were in.

His stomach burbled and rumbled. He felt a growing warmth spread out through his limbs, to his muscles and flesh… and… parts…

‘Uh-oh.’

“T-twi?”

“D-did you just…” Twilight half-groaned, her breath catching in her throat, cutting her off.

“I think…” Shining moaned as he felt the straps begin to bite more into his throbbing muscles.

“...g-get bigger?” Twilight finished, with a gasp.

“...the potion’s kicking in.”

“O-oh… BUCK.”

Author's Note

Oh... oh no.

I had a lot of fun with this chapter... I tried to over-use dialogue as much as possible to give it a more comedic air and let you guys fill in more of the details as to what was going on.

But now it's time to get to business.

BIG BUSINESS

Throw me that like if you're enjoying!