If u have personal belongings there(some clothes hung up) and a piece of mail you can show the officer, the officer will tell his Mom she needs to go through the proper eviction process w/the court and that you have a legal right to be there. Fact. If you do not have any of those things, when the cops arrive, someone needs to en site that says they dont want u there. Your boyfriend could easily say he's on the lease and his Mom is off her rocker which is why she's not there. So, in that scenario, nothing. People have the law confused. U can call the police just because u get mad, but there needs to be an illegal act to arrest you. There first time out isnt going to lead to you being arrested as #1, you were welcomed in, did not break in. Even trespasssing requires some type of warning, hence the "no trespassing" signs. As long as your boyfriend keeps letting u in and she remains away, police will tell her it is a civil matter(money/court) not legal(criminal/court).