"So this is what my dick looks like from this angle," Evan gushed as he grabbed his former balls and stroked his rapidly hardening former cock with his sister's nimble fingers."E-Evan!" Samantha squeaked in shock.She couldn't pull away because Evan had her by the balls. And when he opened her pretty mouth and swallowed his former cock she stopped even trying. She moaned as Evan sucked his own dick with his sister's lips, swirling around the head of his own cock with his sister's tongue. Samantha began sighing, thrusting gently toward her former lips as Evan sucked hard, driving his lips down his own shaft and filling his mouth with his own cock. Evan let one hand drop between his legs, fingers tickling his sister's sensitive pussy as he grew wet.Evan had swapped their bodies but pretended to be just as surprised as Samantha when it happened. He'd been jealous of his hot blonde sister her whole life and had long been preparing to steal her body with magic. Now he had it, he wasn't going to let it go. If she fucked him in their swapped forms the swap would be permanent.He kept her right on the edge as he fingered his slick pussy, sliding into his sister's canal and stroking long, languid strokes while he sucked himself off until Samantha was begging to cum. Then he would lie back and welcome her inside.Sucking his own dick was a small price to pay to wear his sister's pretty face and her perfect body for the rest of his life.

A man gets taken over by a thing from another world that feeds off the chaos it causes from swapping bodies and body parts, transforming people, and then commanding them to enjoy their new forms. Enter the Stranger is too dark for Amazon and available only on Body Swap Stories or Smashwords.
https://bodyswapfiction.com/enter/?p=13336