You roll your thumbs over “his'' little mosquito bite tits, beautiful tiny buds just barely perking up to say hello. The little thing's chest is getting so puffy thanks to the pills you've been slipping her for the past few months. You're worried mom might've caught on, but if she has, she hasn't said anything. That means it's ok, right? I mean, she was obviously miserable being a boy. Really, if she ever finds out, she’ll probably just thank her cool big sis for helping her out! She cries so much more often, and she laughs a lot more, too! All of her emotions have been so much stronger recently, it’s such an adorable change of pace, she was so broody when she was a guy.
And she’s gotten so pretty, too - She was always a little looker, but now her skin was softer, her features were richer, and her hair is getting longer by the day. Maybe that was the real reason why you did this. Puberty was coming for the little thing fast, she probably only had another couple years left of being tiny and cute - the pills were just a way of keeping her desirable! Why would anybody want to grow into a big, gross man? This was gonna get her free drinks and endless compliments for years to come. You’re doing her the favor of a lifetime. Hopefully within a few months you’ll be able to talk her into trying on some skirts and dresses…
You pinch her chest lightly, and watch her legs curl as if instinctively trying to defend herself. She used to be silent when you played with her nips, but now she’s squealing, moaning, whimpering your name like a broken-in slut. The way she wiggles her little butt when she’s needy is new too, and god it drives you fucking insane. This time you squeeze her chest harder, pinching as tight as you can with two fingers and twisting them till she screams.
That’s the breaking point for you. You wrap your lips around her neck and bite down with all you’ve got, printing teeth marks into her soft pale flesh. Your hands grope her punished chest with all your claws digging into her. All the hard work you’ve put into conditioning her to associate pleasure with pain is about to pay off. She whimpers the way she always does when she’s about to have a misunderstood orgasm, and just after she’s ruined her cute pink pair of undies, you pull your face off her neck, revealing the dark red and purple mark you’ve left her.
While she recovers, you slyly slip your hand around her waist and pull down her underwear to assess your work. You inspect the splattering of cum, and are happy to see she’s already jizzing noticeably less than she used to. More exciting, though, was how clear it was. Only a few white strands of semen remained, proof of her declining testosterone. Mom has always wanted a daughter. You had always wanted a little sister. She had always wanted to be a little sister, even if she hadn’t realized it. You’ll watch her progress, adjust her doses as needed, and one day, she’ll realize how grateful she is to have such an amazing big sister.