Mother-in-law Hell

My mother in law has horrible issues! She is bi-polar with a load of different title of illnesses. So she is fifty who is definitely in teenage denial. She dresses like a teen and thinks the world owes her something. I know hurting people tend to hurt others, but this has been going on for quite some time. So, when me and my husband first got together she loved me, you know that conditional type? Well anyways, I heard horrible one sided stories about how she hated my husband’s ex and also how she hates my sister-in-law. Should of took that as a warning sign. Her husband even bows down to her. So, when my husband was with his first spouse, his mother did not talk to him for over six years…
b/c he stood up to her and did not want to take her crap. We have a child now that has spent so much time with them, I can see how her love for him has begun to change. Its so conditional and not sure if its real.
She is interfering with our marriage, like the other day she had the nerve to ask my hubby, her son, if I had made him cry. So here is the big prob, we had borrowed their vehicle for 3 weeks, so when she comes to my house, she is pissed b/c I had drove (her) car. It was never told to me that it was off limits to me. Then she had the nerve to tell me that I had disrespected her and that she demands that I respect her car!
I humbled myself and apologized and tried to explain. I even told her we were willing to help with the car payment, she said she didn’t want our money, and its not about that! So while that conversation was taken place, my husband was renting a car on-line and giving the car keys back to his dad with a check for over $200.00 to help out. So me and his mom go upstairs to check out what everybody was doing. So, she ask my husband what he is doing and so he tells her. Then she runs back downstairs, grabs her luggage and goes out the front door. WE called and called and called. She went all the way back to Huntsville to where she lives and left her husband at my house when we live in Nashville.
She took all of his med’s and he has a heart defibulator. She is so selfish. Then the next day she tells us she wants to come back up and expects us to waste our gas and go and get her! How rude! She knows the only time I get to spend time with my husband is on the weekends due to school and work. So, she comes back up the next day and hardly steps foot in my home. Didn’t speak to me, just kept her shades on the whole time. How do I handle such a person? My husband doesn’t speak up to her b/c he is afraid to lose her again. But, is it worth having difficulties in our marriage just to keep her around??