"No, no more transforming. I want you, Jeff. I want you to love me as you are."
"Very well, little sister. Open your legs."
She did, and he carefully put his penis, which was already erect, against the opening to her vagina. It took him some time to push it in, but once he got inside, it took only one hard thrust for him to break her hymen. Blood stained the tip of his cock as he began to move around.
"Jeff, keep moving. It feels really good!"
Whatever remaining reservations Jeff might have had about fucking his sister disappeared when he realized that, for all intents and purposes, she was a girl and he was a boy. Whether or not they were biologically related was not important. Every instinct told him that he needed to release his cum inside her so that she could reproduce.
"Chelsea, I want to cum inside you."
"Again? You've already knocked me up once. Now you want another kid?"
"Our child needs siblings. And besides, I don't think I can pull out. You're too warm and comfortable. I can feel you trying to squeeze my dick harder."
"Fine, but only this one time. And there's something we need to clear up."
"What?"
"If we're really doing this, if we're really going to raise our incest babies, then I don't ever want them to know that we're related. It would be too much for them. Swear to me you'll never tell them the truth." She emphasized this by suddenly grabbing Jeff's balls and tightening her grip. "Swear or I'll take away your nuts."
Jeff didn't see that he had much of a choice with his balls in her hand. "I swear I'll never tell them."
"Good. Now finish up. We need to start preparing for our babies' birth."
Jeff released his seed inside her and Chelsea calmly got up and dressed herself. "Go put your clothes back on. I need you to pick up these things from the store."