The next morning, Jeff slept a lot longer than usual. At around noon, Natalie came into his room.
'Jeff! Do you plan on waking up today?' She said; but Jeff kept sleeping. She was about to start shaking him when she saw a strange device on his desk. Examining it closer, she saw a label that said 'Chronivac 4.0', and decided to try it out.
When she turned on the device, she saw a complete description of herself. The device somehow knew everything about her, from her hair color to her sexuality. Shocked, she decided to take a look at Jeff's entry, and to her surprise, the device knew everything about him too. She moved the mouse, accidentally clicking on Jeff's hair color, changing it from blond to brown. She turned around to look at him, and to her shock, his hair had changed color.
'What the hell,' she whispered to herself, facing back to the screen. Suddenly, she had an idea. With a smirk, she made it so Jeff would slowly turn into her chubby sister over the next few hours. Better yet, he would notice the changes, but everyone would treat him as if he was always a girl; except Natalie and Sarah of course. She excitedly hit apply, and waited for Jeff in the living room.
After a few minutes, Jeff walked into the room and gave Natalie a kiss. He groped her breast as they locked lips, and after a moment, he sat down next to her on the sofa. Already, Jeff was slightly shorter, and his hair was slightly longer. They sat there for a while, watching TV as Jeff continued to shrink, not noticing his changes yet. His belly began to push his shirt forward as Natalie watched his clothes get tighter and tighter. Her pussy began to dampen at the sight of her muscular brother becoming a fat girl.
Eventually, Jeff got up to go the bathroom. When pulled down his pants he couldn't believe his eyes; where his cock once was, there was now a thin slit between two thick thighs. He rushed to the mirror; his face was now much more feminine. He pulled up his shirt, and found that his six-pack had become a pudgy belly, and his chest had expanded into two enormous breasts. Jeff realized that he was becoming a girl, but why? Suddenly, Natalie and Sarah walked into the room, shutting the door behind them.
'Hey bro,' Natalie said. 'Or should I say sis?'
'Did you do this?' Jeff said, covering his mouth at his newly feminine voice.
'Yep!' Natalie said, moving to grope her new sister's breasts. 'You like it, right?' Jeff was about to protest, but he had to admit, he was beautiful.
Natalie and Sarah began kissing him and touching his new assets, causing him to let out a feminine moan. He felt a finger slip inside his new pussy, sending a wave of pleasure throughout his voluptuous body. Soon after, he experienced his first female orgasm, which made him realize that being a woman was actually pretty great.
Redressing, they stepped out of the bathroom and went to the living room, where their mother was sitting.
'Morning, Jen,' their mother said. Jeff did a double-take when he heard his mother call him by his new name, but he soon got used to it as he settled into his new role. Soon, he would begin thinking of himself as a chubby teen girl named 'Jennifer.'
Jennifer's new body didn't stop the sibling's incestuous escapades however, in fact, they had sex even more than before. Jennifer loved the feeling of her sisters groping her fat tits as they licked her pussy. Natalie invested in a strap-on, and showed her new sister the joys of being filled by a huge cock. After this, Jennifer decided she never wanted to go back to being a boy.