I'm very sorry you have to go through this! You are a very warm, giving person, and do not deserve to be bullied by anyone.
As I was reading your post the very first thought that went through my mind was restraining order, and I still believe that could be the only way, especially if your health is being affected. My only concern with you involving the police is your relationship with people who matter. Don't pay any heed to people who might not see why you're taking this route, if that's the route you choose, they are insignificant in the grand scheme of things. My suggestion is that you let the people who matter, your own family and your sister's family that you want to remain on good terms with, know what your plan is and why you are thinking of seeking the help of the police, before you take the first step. That way they aren't surprised and they fully understand your reasons, that this is affecting your health, prior to police involvement. I'm hoping this could salvage all your relationships with the people who are important in your life, and truthfully they might actually be relieved that you plan to do this.
Just my opinion, please if anyone disagrees with me, I won't be offended at all, I want the very best for momfeelinglost.