My father is cheating and abusing my mother.

My parents got married in 1997. I was born in 1999 and my brother in 2002. My parents moved to some other place for business, leaving me at my grandma's place at the age of 7. My family was never happy because of my father. He never respected any of us neither us nor our feelings, he is very irresponsible, short tempered and do not consider us as human beings. My mother worked like a man to run the family. He used to have an illegal relationship with other lady and abuses my mother verbally like anything for no reason and always doubted her. He maintained that relationship for a period of 10 years and had two children with her. After that she died because of a fire accident. Those babies became orphans. Within no gap he had another relationship with some other lady for a period of 3 years. After that he had another relationship with my mother's sister [ far relative ] listening to her he again started doubting my mother and giving life threatens that he will show hell to my mother. He sent my mother to her brother's house and is not allowing her to come home. He said that if she comes back he will beat her and show her torture. He took a promise from us that me and brother should not call her or answer her calls. We are fed up with words , behaviour and attitude. I suggested my mother to lodge a police complaint and take divorce, she is in a confusion to what to do. So, please give me an advice to what to do to save my mother from him.