Celestia shifted in her bed stretching her legs and wings. She rolled on her back and wiggled her hooves in the air above her as if to bat away sleep's frail hold. Celestia slowly looked down to see an all too familiar tent in her bedsheets. Sighing, she turned to stare out the window into the still dark of night. She had been dreaming of Luna, her sister.
The princess of the sun huffed and rolled onto her side, slowly cocooning herself in the sheets, continuing to gaze at the stars outside. Her internal clock informed her that she still had an hour until she needed to raise the sun. Her cock informed her that she had plenty of time to take care of it before then. Celestia pushed the thought away, or tried to at least but every pulse of her heart reminded her that she was still fully erect and her more feminine aspect parts where uncomfortably engorged. It was for the best that she do something about it now rather than let it fade into a subtle all day distraction in the back of her mind.
Celestia slowly untangled herself from the bedsheets and crept out of her bed, an enormous piece of furniture even by her standards. A simple cantrip silenced her hooves as she made her way across the polished floors and into her private bathroom. Golden light enveloped the bathtub faucet and multiple streams of steaming water started filling the massive tub. A few oils and ointments added swiftly filled the room with a thick mist of soothing scents that helped ease Celestia's mind before she'd even set hoof inside the gilded tub.
Without another thought Celestia stepped over the low threshold of the tub and down into it's recessed basin. She sat and gently rolled onto her back as the waters slowly rose around her. For a moment she had hoped that her swollen shaft would have shrunk while she prepared the waters. Instead the two foot long stallionhood seemed even more eager to perform than earlier. The sensation wasn't helped by the dense weight of her balls, each the size of a cantaloupe, pressing against the sensitive mound of her vagina.
Closing her eyes Celestia inhaled deeply, warm currents lilac and apple blossoms mixed with the slight scent of her own light musk. Wrapping a wingtip around her shaft she began slowly teasing herself. Clear precum dripped from the head of Celestia's cock to fall onto her bare chest as she began to recall the fragmented events of last night's dream. In Celestia's head she could see Luna's dark fur contrast against her own. Feel her sister's wings against her flank. The heady scent of dusk and rain that clung to Luna. The taste of her lips.
Celestia bit her hoof to keep from moaning aloud. Her breath quickened as she began to stroke herself faster. Celestia used her free wing to tease her balls and vagina adding to the pleasurable sensations. She could feel herself getting closer and gasped out her sister's name.
"It is good to know that we are on your mind even whilst awake sister dearest." Luna said from the doorway.
Celestia's eyes snapped open and she froze like a rabbit before a wolf. Even the water in the tub seemed to have stopped splashing. The sisters stared at one another, Celestia's eyes wide with horror, Luna's glittering with amusement. Luna walked across the bathroom with the casual grace of thousands of years of practice and stepped slowly into the tub with her sister. One wingtip dipped into the steaming waters as she crossed the oversized bathtub.
Despite being caught by her sister, the object of her fantasies no less, Celestia's cock refused to rest; in fact, it seemed to throb with an ever greater need as Luna drew closer. Celestia swallowed hard as Luna walked right up to her and, with a stunning ease, lowered her muzzle and drew her tongue right up Celestia's shaft from balls to glans.
"You were late raising the sun, Sister. We came to wake you but found you had already gotten up," Luna eyed her sister's cock a look of unadulterated lust on her lips, "in more ways than one."
Luna kissed the broad tip of Celesti's cock and drank in the sweet, light-tasting pre that poured from her sensitive shaft. Celestia felt her sister's wings brush her own aside. In her wildest fantasies she'd never actually believed that Luna would return her incestuous desire. Yet here she was watching in shocked silence as Luna kissed and licked every bit of her leaking tool.
In a sudden rush of movement, Luna pressed forward and kissed Celestia full on the lips. Celestia moaned as her sister's tongue slipped into her mouth along with a decent portion of her own preseed. The sisters' cocks jostled for space between their bodies as Luna's own pre dripped onto Celestia's chest and mixed with hers.
The familiar tingle of magic wrapped itself around Celestia's cock as Luna's horn lit up. The magic slowly pulsed up her shaft, squeezing it and massaging the massive package. Luna stood up a bit, breaking the passionate embrace of their lips, as she magically jerked them both off.
"You always were the bigger sister Tia." Luna spoke as her magic pressed their dicks together, Celestia's own mast only marginally larger than Luna's. "But we have grown since our return," Luna made short bucking motions grinding their cocks sensitive bellies together, "and learned many things."
Luna's magic pulled Celestia's cock back, dragging the head of her penis across Luna's balls and pressing it against her soaked pussy. Luna rocked her hips, grinding her slick mare sex against the head of Celestia's shaft. The princess of the night sank lower with each gyration, slipping that huge pole in and out of her. As her sister approached her medial ring Celestia gasped. The sensation of her sister's velvety hot passage wrapped around her cock shocked her back into the present, then Luna sat all the way down, hilting the massive shaft in one motion.
Tia's eyes widened in awe as her sister's crossed in pleasure. Her cock made an obvious bulge in Luna's belly that shifted as Luna ground her pussylips into Celestia's crotch. Reaching forward with her hooves, Celestia took Luna's thick cock in hoof and began to stroke it. She kissed her sister's cock, licked all around the flat head of that darkly-colored equipment, gently nibbling on the glans before finally sucking as much of it as she could manage into her mouth.
As Luna started to pump her hips, each thrust pushed her cock in and out of Celestia's throat. Soon their rhythm began to stutter as both reached their peak. Celestia's balls pulled tight against her body and the head of her shaft flared inside Luna. Using her magic, Celestia pulled Luna down onto her crotch and moaned around the head of her sister's cock as she unloaded her massive balls straight into her sister's womb. Luna responded in kind as her shaft pumped rope after thick rope of hot sweet cum into Tia's maw.
While Luna's balls were only about the size of grapefruits, they were easily as productive as Tia's and soon Celestia needed to pull back to breathe. She caught two pumps straight across the face before diving back onto her sister's spasming shaft to continue drinking. The scent and taste of Luna's cum drove Tia to even greater heights. Several minutes passed before Celestia felt herself beginning to flag and Luna's torrent slowed to a trickle. Celestia let Luna's cock fall from her mouth, the last few shots splattering across her chin, neck, and chest. Luna did not immediately rise off her, instead remaining seated on Celestia's cock.
Both sisters looked as if they'd spent an afternoon at an Apple family reunion, their bellies rounded by the intense volume of their divine production. They sat panting in the afterglow,staring at each other. Luna leaned forward and licked a bit of her own cum from Tia's muzzle. Tia kissed her full on the lips, a huge smile on her face.
"Luna, I love you."
Author's Note
Sorry, this one hasn't seen any editing yet. If you see any errors please let me know and I'll try to fix them. Enjoy. 
Some edits made to correct spelling grammar and make things a little easier to read.