Nobody commenting on the sister casually mentioning to him that she erased his memory of her slipping and landing on him while getting out of the tub and she would do it again if he remembered?
So she can manipulate memories, and he's blindfolded in the bath, theoretically to keep him from seeing her, but she's also the one bathing him so that he's not feeling himself. Maybe he's not really genderbent in the real world and she's manipulated his memories to make him think he is, so she can't let him feel himself or he'll figure it out?