Mental Unstability. Suggest what to do!

My brother got married 4 years back with this girl from newspaper matrimony add. After the marriage we started feeling that the girl is mentally unstable from her activities. Her parents never let her stay with my parents for a long time. She gotten these mood swings. She would be very normal at a time and get aggressive the very next moment. Somehow my brother could not figure out about her condition and planned a baby who is about 2 years now. For almost a 6 months she lived my parents after her delivery at her parents place. My parents observed her well - she used to make the 4 months old baby bath in open at night during chilly nights with cold water so many times. She attacked my dad to bite his hand once. Since it was about the baby, now my mom lives with my brother and her which she is does not like. She beats my mom for no reasons every now and then. She is getting violent as days are passing. She does not even listen to my brother. Kindly suggest what to do