Your sister turned you into a girl for being a lazy selfish horrible brother. You are her little sister now. You threaten to tell both of your parents and she smiles smugly and says

"Go ahead and tell them, 'bro'... oh I mean 'sis'! With how you acted as a boy, I don't think Mom or Dad is going to make me change you back. I'm sure Mom will love having another daughter, and well... I'm sure Dad won't complain..."

She smiles smugly at you, and says 'bro' sarcastically, you are her 'sis' now.