Kat quickly had a great idea. When they'd been younger, Dan had been proud of being a boy to a really stupid degree, treating it as some reason to think he was better than her even though it meant nothing more than that his body had developed a penis instead of a vagina before he'd been born. The more she thought about it, the more the idea of having a sister appealed to her, and she couldn't keep herself from wishing for Dan to change sex.
Inside the egg, suddenly Dan could see, and he felt a pang of relief at this. But as he started to feel strange, he realized it wasn't Kat suddenly being nice to him. He could tell Kat had wished for something, and suddenly, she wished again, and his clothes vanished.
Unable to move to protect his modesty, he had a full view of his naked form as Kat's wish started to alter the outcome of the developmental coin flip of his assigned sex. All of a sudden, the masculine smell of sweat from the clothes around him seemed to turn him on. His discomfort at the realization was quickly reduced by his brain, though- of course he'd be turned on by the smell of boys, he thought, he's straight, of course he's attracted to boys.
Wait, what?! He... she... wasn't... was... everything felt in a state of flux as his old masculinity and her new femininity clashed, with the wind at the feminine side's back and the odd pleasure it wracked Dan with. It won small victories over his body as his skin softened, his hair lengthened and his muscles slimmed. The part of him that screamed he was a boy slowly quietened, his body and mind became pliable, the assertion of Kat's wish grew stronger as she shifted fate, the coin flip turning from male to female.
And as it did, Dan felt himself start to become Danielle. His cock slowly receded into his body, his balls moved up inside her where they were always meant to be, her chest budded out as she grew soft C-cup breasts, and she adopted an hourglass figure. Between her legs what remained of his penis soon formed her sensitive clitoris, and his scrotum shrank into her small mound and opened out to form her vaginal lips.
By now Danielle's perception of boys was totally disconnected from any memory of being one, and in her current horny state was mostly sexual. She was semi-consciously thinking of handsome guys from her year, and their big cocks, and how much she'd like to suck and fuck them and have them cum in her mouth or on her tits or inside her pussy.
Kat sensed this and giggled to herself- clearly her horny sister was no less of a stupid slut as her horny brother had been. She wondered what would be the most fun way to satisfy that stupid perv now she'd finished changing...