Speaking as both the mother of a student, and a student myself, I think your mother's plan is completely unacceptable. It's not fair on you or on your flatmates for her to stay over on the first night. It's probably against the university's regulations anyway. I suggest that you find out if this is the case, and if it is, then send the accommodation office an email asking whether your mother would be allowed to stay in your room. If they reply in the negative, you can cc your Mum into the conversation, so that she can see that it's not acceptable.