Thousands of stories to choose, and I chose this one
In the then-peaceful Everfree forest, there lived two sisters. The Two Sisters, to be precise- Celestia, coat white as the stars above; and Luna, indigo like the midnight sky- the rightful Diarchy of Equestria. Each respected the other, and were as kind as sisters were able to be. But slowly, as their reign went on, it became clear to each other: both had very different ideas on where to take Equestria, and so one or the other would have to become the primary head of leadership.
Neither wanted to ceede power, but one had to. It also didn't help that they were entering their teenage years, and so their odd mixture of hormones was wreaking havoc on their decision making. When Chrona, the goddess of time, had created the Two Sisters, she wanted them to accurately represent all Equestrias, and so she crafted them with both penises and vaginas in order to achieve what she saw as the best mix. But she failed to foresee what the odd mixture of hormones would do to their decision making, especially in such emotional matters, and so she took it upon herself to intervene before it broke the two apart.
"To decide who shall lead and who shall follow, I give to each a simple task, a competition to complete. One of you must get your cum into the othe; whoever gets their cum into the other, while avoiding get cum in themselves, shall win and become ruler." Chrona offered, to resolve their query.
"A simple contest, sister?" Luna asked of Celestia.
"I agree, a fair one. We shall partake in the event"
"Very well. In five hours, I want it done. You have your duty, now prepare yourselves." And with that, Chrona disappeared into the aether.
"Well, you heard her. May the best one win, dear sister."
Five hours from now, the chamber of the Two Sisters
The Two Sisters had rubbed each with hay oil, as was Equestrian fighting custom, and readied for a duel of strength and carnal pleasure, each using magic to get their penises hard and ready.
"Are thee prepared, little sister?" Celestia asked, giving a sly wink which threw Luna for a small headroll.
"As prepared as thou's holes should be for my member to leave their mark" Luna threw back
"Very well. Three, two, one, go!" Celestia counted off, and soon the fight was quickly becoming one sided-in favour of Luna.
Luna had wrestled her sister into the mattress, taking advantage of her shorter height- and greater stability- to trip her up. And soon, she was on top, with her glistening and oiled indigo cock being sandwiched between Celestia's large flankcheeks. She shifted an infinitessimal amount, just enough to move her cock to the hole of her choosing, and she chose her puckered and winking ass.
"And now, dear sister, I shall ruin your ass, and cum shall flow as though a river" Luna said, leaning a little into her cock.
"Do your best, Lulu, but I shall prevail shortl-yyyyy" Celestia cried out, as Luna, tired of her sister's monologue, pushed forward with her full might. With a swift plap of her balls against Celestia's flanks, she found herself already hilted, surprised at how loose she felt but not complaining.
"I must admit, sister, I had never took you to be of the anal slut variety. How many stallion have thee taken back here?" Luna teased to her
"Buck-" "you-" "Luna" Celestia cried between thrusts.
"A backward presumption, for it is I who buck you. But I shall not waste time with soliloquy. Prepared to be conquered, and for I to be the head of castle!"
Luna sped her fucking to an almost-untrackable pace, clearly trying to end this as fast as possible. She felt almost bad for her sister, being shoved snout-first into her own bed as Luna ravaged her hindquarters, but she found it thrilling. And the idea of filling her ass to overflowing sent extra throbs throughout her body.
But, Celestia was more clever and devious than she had led on. She had perfected a simple teleportation spell, to the point where very little magic power was needed- so little power, in fact, that it gave only the faintest glow, which would be unnoticeable unless close up and paying very acute attention.
She waited until Luna was just on the verge of cumming, so distracted by achieving pure carnal pleasure that she would never notice a faint spell. And, from how erratic and careless her thrusts were becoming, Celestia guessed that-
"Get ready, sister," Luna said, cutting off Celestia's thought with an emphasised thrust "for here-"
"I-"
"Cuuuuum, holy Faust!"
Celestia slowly- slow enough to be ignorable- lit her transportation spell, intercepting Luna's voluminous load right as exited her cock's urethra, and dumping it into a stream just outside the castle instead. Feeling her sister's blasts of cum hitting her magic, she was concerned that she might not be able to catch it all, or even half of it, from just how much she was cumming. She even swore she could even feel Luna's testicles jumping as they pumped each shot as hard as they could, to the point of concerning Celestia that she might over-power the spell through sheer kinetic force, foiling her plan and getting her in a lot of trouble if Luna saw the glow of her horn suddenly die out.
Eventually, the vertible blasts slowed to squirts, and then mere trickles of lunar cum into the celestial ass, leaving the two heaving sisters to wallow in their respective afterglows.
"Well, dear sister, it seems I have won our contest."
"Indeed you have, little sister, and what now shall you do?"
"For now, I think a snack will do. Would you mind retrieving one?"
"But of course, you have won after all. Zap apple, from the bushel we harvested earlier"
"Yes, please, that would be most splendid"
Celestia smiled deviantly, on the inside, knowing what she did to the apples earlier. "As you wish, sister, I shall have them up in a moment"
Two Hours Earlier
Celestia thought frantically, how she would win and get her cum inside Luna. She knew Luna would easily best her in pure strength, so that was out of the question. She looked smaller than Celestia, but she was a brick wall when she needed to be.
'But what else can I do?' Celestia thought, 'it's not as though there is any other way to get cum inside somepony, is there?'
She pondered the question before realising: it was never specified which hole the cum had to enter, or how it got in there, just that it went in. And that's when it dawned on her: Luna loved snacks, of all things from fruit to hay chips, and especially after sex. And, through sheer coincidence, she and Luna had harvested a bushel of her most favourite of all: zap apples. The little chromatic-patterned fruit were nigh-irrestible to the lunar one, it would be unlikely that she'd inspect the fruit too closely before eating them. Which gave Celestia the perfect plan: slice up the apples, cum on them, and then wait for her sister to go for her snack.
And so that is what she set out to accomplish. She sliced up two apples, and then she hid away in a quiet corner of the house.
"Come on, come on" she chanted, as she yanked her princess penis with her warm magic. While it was, outwardly, purely a strategic move to win, Celestia couldn't deny that she found considerable pleasure in just the thought of the act. It seemed so deceptive and depraved and disgusting- and she loved it. Imagining her sister, licking and sucking each apple slice clean of her jizz before then plopping it in her mouth, chewing slowly to savour the flavour of such an unorthodox combination; it was such a foul act, and a thrilling one, and which was speeding Celestia into her climax.
"Almost-" she choked out, pumping faster along her shaft and swinging her hips along with each stroke "there..."
A thought of Luna sucking each apple only for the cum, and then discarding the slice, is what brought her over the edge. Her star-white balls flexed into her crotch, pushing their prize forward and shooting along Celestia's urethra. She was careful not to make too much noise, lest she arrouse suspicion from the castle, but silently she came, and directly on her targeted platter of apples. Soon each apple slice had a coating of her cum, almost resembling a glaze from a doughnut, but she still hadn't finished cumming, so quickly she brought her mouth down and other her glans, drinking in any extent cum. Ironically, this only made her start to cum harder, finding another odd pleasure in this act too, but eventually her balls calmed themselves enough for her to pull her mouth away.
And now, all she had to do was put the apples back and wait...
The current moment
And her waiting had paid off. The apples sat undisturbed, and the time difference allowed some of the cum to truly seep into the apples- giving them a sheen instead of looking like glaze. Quickly, she grabbed the tray and dashed back to her room and waiting sister.
"Thank you, sister. These shall be of great use as I recover my strength" Luna said, taking two slices from the dish and biting into them, unnoticing the texture. "Hmm, these taste different than I remember. Oh well, I do suppose it has been a good few moons since last these were in harvest"
"Indeed, I suppose so" said Celestia, doing her best to avoid a sly grin giving her away.
The morning after
"Step forth, each sister, so I may scan thee for who won" Chrona said.
"I believe it be unnecessary, mother, for I had bested Celestia quickly and won the contest" Luna said, with the sort of grand overconfidence gained through victory.
"Be that as it may, I shall still need to scan each of you. It is the rules." Chrona replied to her.
"Very well, scan me first" Celestia said, stepping forth with a determined poker face- the kind which slid past Luna like an otter, yet Chrona recognized instantly as hiding a trick.
Chrona scanned a sample of each's cum for reference- Luna's glowing green and Celestia's emitting a beautiful pink- and then scanned Celestia, finding nothing.
"It seems she is clean." Chrona said, puzzled but curious to what Celestia done.
"Wha- but, that's impossible! I came buckets into her, where has it gone?!" Luna said, shocked into pure disbelief. She lit her horn and used to same spell, finding the same answer.
"Luna, please step forward to be scanned" Chrona said. Luna, too shocked to announce any protest to the command, did as instructed and submitted to having herself scanned for her sister's cum.
Almost like turning on a lava lamp, Luna's stomach glowed the very bright pink. Celestia couldn't hold her face any longer, and broke into a smirk a mile long.
"Gotcha, Lulu." Celestia said, lighting her horn at the nearby stream, which soon shone green as the emerald grass. "It was in the zap apples", she added to her testimony, soon laying out every detail of her plan to the two, one with a face of disgust and the other of indifference.
"While unorthodox, you did fulfill the rules of the contest," Chrona said. "Celestia, you are to be the head of the Castle of Two Sisters" and with that decree, Chrona left once again, the two sisters now to face only each other.
Author's Note
"And that's why Luna became Nightmare Moon!" Pinkie Pie said to her campfire audience, with Rarity, Applejack, and Rainbow Dash each covering their respective sister's ears as all six mares stared in awe and shock at the party pony.
