Your feelings of frustration, disappointment and disgust are not only valid, they seem absolutely reasonable and proportionate.

He's your mother's ex husband? Sounds like she feels obligated to keep him around and act as a support to him. Are you obligated to maintain contact with him?

The guy is a more than a little unstable (sane adults know not to joke about kidnapping children) and has more than a few fetishes that he likes to involve other people in, unwillingly. If you feel unsafe around him at any point, you should trust that feeling. However, if you feel ok, that may be genuine too. Your gut is pretty reliable, listen to it and not to what others think you should do. But even if you feel safe around him now, if you ever do feel fear act immediately. Don't hesitate or second guess it. Get yourself to safety, even if you have to break a window, run to a police station, and cause a scene. Your safety is more important.

It's wrong that he took your sister's name. It's a violation and (gross) it's part of his fetish. He wants to be a baby, and a girl, and he named himself after a girl he's known since she was a baby - that is not a coincidence. It's wrong that he's trying to rub it in your faces.

You're allowed to not spend time with people who make you uncomfortable. Not everyone is worth your time. However, it sounds like he probably comes around a lot and isn't someone you can easily avoid. Can you tell your mum that's he's making you uncomfortable and that you'd like to limit the amount of time you two have to spend in the same room? Maybe she can give you a head up when he's coming over and you can just happen to be at a friend's place? If not, I suggest:

• Give your sister a rad nickname like "Ace" or "Bandit" and call her that so he can't rub it in your faces that he's taken her name.

• Don't talk about "girly" stuff in front of him, and if he brings it up say "Eh, I'm not really interested in that."

• Point out when he's being overtly sexist (if you feel safe to disagree with him). It's ok. It's good practice for later in life. Eg. "real girls have to watch their weight" "I don't know, that just sounds like 50s sexism to me. Young women need to fuel their bodies too." or just "sounds a bit sexist."

Right now he's getting a "fix" from being "girly" with you. Cut off that supply.

In reality women and girls do everything that men do. We eat, we wear boxers, we get dirty, we play sport and we fart. Don't let him make you feel wrong for being a full person and not a cartoon.

Look up the Grey Rock method. This guy is deliberately making you uncomfortable in a situation he knows you are stuck in, because he likes to watch your discomfort and make you do what he wants. You're his little puppet. Whether you go along with it, ignore it, confront it, or get uncomfortable and try to avoid it, he will enjoy that he made you have that reaction. So be the grey rock. The grey rock has no strong opinions about anything. It doesn't react. It can't be teased or provoked. It's totally uninteresting. If you go the Grey Rock method, everything he says can be responded to with a half shrug and "Eh. Ok." It'll be written off as you being a moody teen but when the game gets boring enough he'll stop playing.

BTW "medical tests have shown that he is 80% female" is once of the most ridiculous things I've ever read. Brittle bones doesn't make you a woman either.

Good luck, it's going to be tricky to navigate but hopefully you'll be grown and out of home soon and you can decide who you spend your time around.