Why do some mother in laws treat the daughter in law bad?

My husband and I have been together two years. We were recently engaged. His mom has always hated me. I am in college with a 4.0. I have a job. I make her son happy, and we basically believe in the same thing religion wise except she is catholic, and I was baptist. She always yelled at me and told me that I will never be before her. She has two other sons, but she messed up her relationship with them, and they never talk to her anymore. She kicked my husband out when he was my fiance because he made her boyfriend mad. She is married to another man btw who loves her to death. Well he came to live with me. After five months, she wants him to come back "home." He refuses, so she calls the cops four times and tells them that he is mentally unstable and is like an eight year old and told them to make him come home. He is about to be 21 also. So the cops get tired of it and tells us to place a restraining order on her. We both have nothing to do with her now. I just do not understand why she hates me so much.