Jeff was always sort of into weight gain, but he never told his girlfriend about it, fearing that she would think it was weird. Now that he had the Chronivac, he could finally fulfill his fantasy. Using the Chronivac, he changed Natalie's mind so that she would find weight gain hot, too. Then, he made it so that when they kissed, Natalie would slowly gain weight. Hitting apply, he went back to his sister laying on the bed.
Leaning over her, Jeff pulled his sister into a deep kiss. Immediately, he felt her expand outwards slightly, her breasts growing larger, and her midsection becoming slightly chubby. Breaking off the kiss, he placed his hands on her now larger belly, his dick hardening at the feeling.
"Woah. When did I get this chubby?" Natalie asked, grabbing her stomach.
"I think you look good like this," Jeff replied. After a moment, Natalie agreed, blushing.
"This might sound weird but... I wish I was fatter," she said.
Jeff began making out with her again. He massaged her growing breasts and belly, his dick now rock hard at the sight of his sister inflating like this. Her torso exploded outwards, adding a few rolls to her stomach, and a few cup sizes to her breasts. Her sweaty thighs became chubbier, and her ass grew to an enormous size. She had to have been at least double her original weight, Jeff thought.
Again, he broke off the kiss. Natalie looked down and gasped.
"Woah! This is definitely not normal," she said, exasperated. She stood up, her entire body jiggling with each movement. Looking in the mirror, she examined her new, plumper body.
"Did you do this?" She asked. Jeff showed her the Chronivac, and how he made her grow with every kiss.
"Jeff, I love it!" Natalie said. "Can we fuck now?"
Jeff took his cock out, and pushed his sister's fat body back onto the bed. He rubbed his member against her lips, making her moan. Slowly, he began fucking her pussy, sending ripples through her plump thighs and ass. Jeff grabbed onto her love-handles, and thrust into her faster and faster. Both of them moaned in unison as they climaxed, Jeff's cum filling his sister's pussy.