YWNBTA. Look, you literally don't have to move this bunny in with you. People are saying "well you can't plan out everything..." blah blah blah. That's bullshit. Yes, you can decide whether you have pets in your own home or not. Tell the step dad to return the bunny. I'm certain whoever/wherever he got it won't turn down a returned bunny. It's bullshit she is imposing this on you like neither of y'all have agency. You can either let her walk all over you and keep the bunny, or stay firm and tell her it's not coming with. I'm telling you, if you let her streamroll you on this, you'll be seeing this dynamic play out a whole hell of a lot more in the future. Choose wisely.