how do you get along with your MIL?

I've been with my boyfriend for seven years me and his mom have a love-hate relationship kind of. she has a big mouth and a bad attitude and Im bipolar the difference is I'm not afraid to tell her to stfu like everyone else is. She has lived with us for almost 4 years now she has no respect, she is dirty, she is disgusting, her room looks and smells like shit and she doesn't pay rent she just recently started paying PG&E. We have tried many times to kick her out but it has never worked she always came back so we just decided that it was time for us to leave. We are buying a house this year so that leaves her and my boyfriends brother to pay the rent and the rest of the bills in the house, which she hasn't had to pay more than one bill in 4 years! She has a job and she's been working for almost 2 years so does his brother so we're not leaving them high and dry. Anyways this morning I washed a towel, left it in the bathroom hanging over the shower because I was gonna use it; I told her because her bedroom is attached to the bathroom. she had to work so she jumpes in the shower first after She is done I walk in to take my shower and my towel was gone; I asked her "are you using my towel?" She replied "I'm using a red towel" so I said "yes you're using my towel" she opens the door and handed to me saying with a smart ass attitude "well actually it's belongs to the household house" I responded "I washed it, I hung it over the shower and I planned on using it so therefore it is actually my towel and i told you this morning" she shut the door and i can hear her making stupid comments so i called her Smart ass! She seems to think she is the only one in the house that has a say or is allowed to have an attitude or is allowed to make stupid comments to people but when it's done back to her she throws a fit and gets an attitude! I'm just ready to move!!