My boyfriend's siblings want to move in with him but I don't want them to.
That's weird. Why don't you want his siblings to move in with him?
It's too crowded, and I don't like his siblings.
It's not your business if his family moves in with him. You should be a lot more understanding if his siblings need a place to stay.
It's my business because I live there too.
You do have more of a voice since you do live there but understand that if they have a need, your boyfriend will feel bad not being there for them. You might also need to help a family member in the future and you wouldn't want your boyfriend to stop you from helping them.