Your sister finally rolls off of you and falls on the floor next to you. Her huge, enhanced breasts heave like she's just run a marathon. You lay side by side for a couple of minutes, before she props herself on one elbow and starts petting your chest.
"I don't know what you did to me, brother," she says, "but I like it. And I like the feel of your naked cock inside me."
As she says that, your enhanced penis jumps and twitches. You look at your sister. She still has an innocent sixteen-year-old face and lustrous black hair. But this morning, she was a skinny beanpole with scarcely any breasts or ass, and now, she's this voluptuous goddess. A few hours ago, you shared your house with a little girl, and now, she's very much a woman.
"I was a virgin before you... jumped on me," you say.
"So was I," she says, and giggles. Her laugh makes her tits jiggle just right. You feel yourself getting hard.
"No," you say, though even you don't believe it. "Brother and sister taking each other's virginity? That's just wrong."
"Oh, no, brother mine. Don't try and get all moral on me after you sculpted me into your big-breasted ideal of a perfect woman. If you didn't at least slightly want me, sexually, why did you test out your breast gun on me?"
"Because you were convenient."
"I don't believe that any more than you do, brother. You keep asking me to go to the mall with you, and you write me poems on my birthday, and you act like it doesn't bother you to chauffeur me and my friends everywhere. You basically treat me like your girlfriend. Well, that's what I am now. Your custom-designed, incestuous girlfriend."
With that, your sister straddles your hips and leans down to kiss you. As your faces mash together, she grinds her new, massive tits into your chest, and it feels so good. She doesn't have to rape you this time. You grab her hips and raise her up slightly, so you can guide your penis back into her.
As you make slow, sweet, unprotected love to your perfect teenage sister on the floor of her bedroom, neither you nor she realizes that an egg has just dropped into her womb.