"Since you're such a gross ass that's what you'll be!" Kat proudly declared. With a flash of green light her wand shot a bolt of energy straight at Dan's chest, he yelped in fear and raised his arms to shield himself, though the magic still hit him a portion of it bounced off the reflective face of his watch, rebounded back and hitting Kat as well, she was to share in his fate.
Kat screamed "oh fuck!, no no no!" as she realized what was about to happen. The wand dropped to the floor as both siblings doubled over in pain and they began to transform. Falling to all fours Kat's ass balooned in size as her legs thickened and her hands and feet twisted into hooves, gasping in pain she watched as Dan brayed as his torso expanded and tore his shirt away, his feet and hands hardening into hooves to match her own.
As they both grew their clothes could no longer accomadate them, Kat's pants soon gave way, freeing her now huge rear, they were soon followed by her shirt as it tore along her back and dropped to the floor alongside Dan's now destroyed jeans. In between flashes of pain Dan watched as his sister sprouted a tail and her ears shifted to to top of her head, elongating as they did, "karma's a bitch sis..." he moaned, glad at least that she was sharing in his fate.
Before Kat could retort Dan's comment she was shocked speechless as she saw his penis begin to rapidly expanded, tearing aside his underwear, its girth as thick as an arm, mottled with a bestial flared head. As an alien lust began to overtake her Kat stared as her brothers swollen balls to distract herself as she felted her vagina and asshole migate up to underneath her new tail.
Unable to control her new instincts Kat turned and presented her rear to Dan just in time for him to see her panties give way as her privates slowed pulsated and expanded, her pink lips turning a dark brown. "I can't believe I'm doing this... fuuu, fucckkk meehawwww" Dan also overcome with bestial desires needed no further invitation as he reared up onto Kat's back and penetrated her with a single thrust.
As the siblings fucked their transformations neared their end. With each slap of Dans balls on her ass Kat felted hairs begin to sprout all over her body, her now swollen breasts shifted down her barreled torso and morphed into teets between her legs, swinging in unison with her brothers balls.
Though what was happening would have horrified Dan only minutes before now he felt elated as he fucked his sister, this was revenge, he would make the bitch his and see if she would be so quick to mess with him when she was helpless and bloated with his cum. Necks stretching they both brayed as their faces began to pull forward into muzzles, "Yourrr mine nooow you bitchehhaawww!" managed Dan just as his transformation ended and with a final thrust he began to nut in his sister.
All reason lost to her Kat gasped as she was filled with Dans virile cum, "knockaaae meee uppaa you studhawwww!" she shamelessly begged between ropes of drool as her she too finished transforming. Now only two Donkeys remained, Kat nuzzled her mate as his cum ran down her legs, though retaining human minds neither of them had any human inhibitions or shame left. Turning Kat presented her dripping rear to Dan once more, she supposed they better go again to make sure their mating took.