As they held each other, awkwardly making out, neither of them used to Sarah's snout, they remembered that Natalie was still home; they were amazed that she hadn't woken up to their loud mooing. Suddenly, they both got the same idea.
Jeff and Sarah walked down the hallway to Natalie's room, their full udders sloshing with each step. They opened Natalie's door, finding her still sleeping through all the commotion. Jeff rubbed his new pussy as they giggled mischievously, climbing on his sister's bed. They both slowly stuck one of their teats into Natalie's mouth. She instinctively began sucking, sending waves of pleasure through their bodies, forcing them to suppress their moos. Her mouth filled with milk as she slept through her transformation.
Natalie's body began to change. Starting from her mouth, her skin turned a dark shade of brown as a pair of horns pushed out of her head. As she sucked their udders, her mouth pushed out into a bovine snout, barring her from eating anything other than plants or milk again. Her belly filled with milk as her bones reconfigured themselves, growing a tail and changing her legs into hooves. Her breasts expanded, ripping her shirt open, exposing her now lactating nipples. The area around her belly began to fill with fat, transforming her formerly petite body into an overweight goddess. Finally, her udder began to grow in, leaving a massive bulge in her pants.
Suddenly, Natalie woke up. As she opened her eyes, she saw her brother and a half-cow half-girl with their crotches hovering above her face. She started to say something, but coughed as her mouth was filled with milk. She slapped their dangling udders away from her mouth as she tried to talk, but all that came out was a cow-like moo. She looked down, finding a fleshy mass between her legs. She noticed her skin had changed pigment from a pale, pinkish colour to a deep, sexy brown.
'Why is it so hooooooooot,' Natalie said, mooing, as she ripped her shirt and pants off, leaving her naked. Jeff grabbed her new teats, and began to milk her, making her moo louder.
'You're a cow now, sis.' Jeff said, smiling. 'You're like us.'
'What? What happened to-' Jeff cut her off by planting a kiss on his sister's new bovine muzzle. She gave in, starting to make out with him as he continued to milk his sister.
'Looooooooks like she likes it, babe,' Sarah said, moving toward Natalie's cow-like pussy. She stuck her bovine tongue inside, making Natalie moo uncontrollably with pleasure. Her cow tail waved wildly behind her as her brother and his girlfriend pleasured her.
Eventually she orgasmed, the whole ordeal leaving her bed utterly soaked with milk.
'I don't know what you did, or how you did it,' Natalie said, panting. 'But I'd be lying if i didn't say I looooooooove it.' Jeff kissed his sister again, as they fingered each other, Sarah rubbing her pussy as she watched the siblings have sex.