Mother in law and husband's ex wife best friends?

My husband's ex wife was never really close to his family while they were married, and when we started dating, my in-laws didn't exactly praise her. Now, she has decided to be involved in their lives. My mother in law told me if it bothers me to not come over if she is there. The ex has hared me incessantly with phone calls and slander and my mother in law knows how much it hurt me; also, the way she treated my husband while they were together caused a lot of pain in their family. There are no children between my husband and his ex so I see no reason for her to be so clingy. It really upsets my husband as he feels his mother is choosing to please the ex over him. She knows it bothers both of us, yet continues to allow this woman in her home with no regards to how it makes us feel. In our opinions, it seems that the ex is trying to tear us apart by being so close to his mother and/or get him back. Why is my MIL being like this? And what do we do about it?