You didn't move out when you were 19, you moved out when you were 25. Do you want to move back in? It's also still been a year before they are moving in so it's not like you were kicked out for them to move in. Are you unhappy since moving out?

She sounds awful and petty but what do you want to do about it? Your dad is aware of what you told him and his gfs reaction to it and still wants to date her. You can let him know your feelings are hurt on how she treats you but I think that's a separate issue to feeling replaced in the home. I don't think you were replaced, I think you don't like them (for good reason) and your angry so it feels unfair. You should have a conversation with your father about how she treats you for sure, but I don't see how her daughter moving in is a slight to you.