Uncomfortable with mom's boyfriend staying the night?

K so my parents finally got official divorced a few months ago, but were seperated for ugly 3 years. my mom's new boyfriend and his duaghter are staying the night more and more and it makes me feel really uncomfortable. I like the daughter and hes not a bad guy it just makes me feel EXTREMELY uncomfortable that hes sleeping over. I've told my mom and she basically says "well there nothing I can do about it, your being completely unreasonable and i dont even understand why'd u'd feel that way". she basically writes me off. I don't think its unreasonable i ask that he not saty over when i'm here (i go to my dads every other weekend), especially as they've only been dating for a few months. Are my feelings justifiable?