You won't get a restraining order against this woman no matter what you do or how hard you try.

The only person that can do anything about this situation is your MIL, and if she doesn't do anything, the law has no choice but to assume that this is the way she likes to live. If your MIL changes her mind at some point and decides to change her life, you may assist her in effecting the change, but that's about it as far as your role in this situation is concerned.