Brendon sat on the couch watching television. His step-sister, Molly came in, a wicked smile on her face."What do you want?" Brendon sneered.She didn't answer, but she pointed at him and uttered some strange words. Then she began changing, growing taller, taking on more mass, her features sharpening. Brendon jerked up from the couch and realized his own body was changing. His bulk thinned out, arms becoming slimmer, fingers daintier, his entire body shrinking. His massive pecs softened and disappeared, replaced quickly with two small bumps. Breasts! Brendon just had time to realize before his face wiggled and twisted and silky hair grew down his head, obscuring his vision. He pushed it out of his eyes and brought his hands to his cheeks, feeling the soft new flesh.He froze then, gaping over at Molly. but it wasn't Molly standing there anymore. It was him! His body, anyway. Brendon gasped, a light, airy sound and jerked his hand away from his face, gaping at his hands. The nails were a bright red, just like his step-sister was wearing. Brendon's clothes dwarfed him in his tinier body."I just wanted your life. Now I get to be the star quarterback that dad loves. And you can be the girl he wished was a boy.""Change us back!" Brendon roared. Or tried to, anyway. His voice was more of a squeak that just made her smile.When Brendon got up to try to threaten her he realized she now loomed over him. She leered down at him and he drew back, suddenly conscious of how small and fragile he seemed in Molly's petite body."Nope," Molly smiled. "Now I'm gonna go try out these muscles," she said, flexing her biceps.She turned and left, leaving Brendon alone in her body. He was stuck like this. No one would believe him. His only hope was to somehow find out how Molly had done it and change them back. But for now, he could only play her part.

A young man finds an app that lets him possess any of his online followers and live a month in their body in Instaswap 1, available on Body Swap Stories, Smashwords, and Amazon. Preview here.
https://bodyswapfiction.com/enter/?p=13388