Julie was sitting on her bed, reading a magazine.
Un-perceptible to her, two growths were forming on her crotch as her body spontaneously rewrote it's DNA in response to the changes made by Jeff. As testosterone spewed forth through her system, her muscle mass changed and her hair shortened. Her female 'assets' didn't change much, although her breasts became more toned through muscle changes, wiry pubic growth bloomed around her crotch and her clitoris snaked an inch out of its hood, taking on a minor penis-like form.
As her memory re-wrote itself, Julie grew bored of the magazine and went to visit Jeff in his room. Maybe she could surprise him and get him in a headlock or something.
As she stood, the testicles pushed out against her panties and tight leggings - she looked in the mirror confused. "Wait, why am I wearing these things... Meh, I'll change afterwards!"
She snuck down the hallway and crept into Jeff's room - he had his back to the door and was browsing the internet. Julie leapt on him, surprising him & catching him off balance. He fell clear off his chair and she pinned him to the floor.
"3:2 to me, little man!" said Julie. Jeff didn't answer, his brain processing the changes that his sister had undergone - were it not for her breasts, he was sure he would be staring at his brother right now.
Almost uncontrollably, Jeff reached for her crotch and felt two lumps - Julie recoiled and shouted at him. "Hey, not fair! You know our balls are out of bounds!"
Jeff stuttered "Err, whuh... Oh, yeah, sorry! Jesus, you're getting stronger. Are you working out again?!"
Julie replied "Of course I am. It's what the girlfriend likes!"
A panic alarm went off in Jeff's head - girlfriend! It was starting to settle into his brain that by changing something so small and, more appropriately, giving it a history, that he had not just changed his sister - he had changed the entirety of history. He'd not only given his sister a pair of strapping balls, but he'd turned her into a lesbian... sort of!