How to save a girl's child from mentally ill, arrogant, mother?

Respected Sirs/Madam, I m Bharati sengupta, working in a co-op bank for last 26 years at Mumbai. This is about my Brother, his wife and their daughter. In 2008 when my Daddy was sick and bed ridden we had ask one Ayaah/nurse from Kolkota who had worked in my father-in -law's old age home. She started in train to come Mumbai my Daddy left the world. Next day the girl reached Mumbai and said to mother that she does not have any body so she would stay at our home help my mother and do same job from here. She cried and hold my mothers feet to allow her to allow her to keep her at her home. Gradually she became ready to get married to my brother and finally with their consent we arranged their wedding after intimating her parents. After her delivery-first baby she started behaving very awkward way. Any time she started leaving our home, going to mother's place for more than 1-2 months by saying that she is going for 10 days. Then forcing my brother to bring her from Kolkata. Once it was ok but it became her regular hobbit. Once she came to Mumbai after a year or more, stayed few days and again disappeared for 4 days leaving a year old baby. After 4. Days came and fought to take her baby along with her and said she will stay with my bother if buys another house. During all this time she fought with my brother, tried to harm my mother physically and by abusing. Looked all cupboards, kept keys with her. Tried to steal imp. Papers and other things. Broke computer, tv, mobile, my brother's spaces. She stopped cooking etc. After trying to make her understand,. We came to know it was useless. We did several police NCs. I brought my mother at my home. Once she was ready to leave my brother on mutual understanding but then again refused for the same. And by telling so many lies took her daughter with her. During all this we all and building people had noticed some mental disbalnce in her behavior. Even she had told my brother that she had heriditary of such mental illness as according to her, her mother used to get some mentall problem and so her uncle used to heat her after her delivery for at least 3-4 months. Even her brother was admitted in Mumbai for mental sickness where even she was advised to get admitted for the treatment. But she was ready. When she tortured her child like heating her, making the child seat on high narrow wall after making the child bald, sometimes wrapping the child in blanket in summer season, not giving her even water when child was thirsty and so many such things, asking building people to give terrace keys, Bldg. people also used to get scared of her. They asked to do something or the child and my brother were in danger of their lives. We took her to a Psychiatrist he advised to admit her in nursing home but the expenses were too high for us so I enquired with few known people and put her in Govt. mental hospital for about a month for her treatment. But she said my brother that she will take medicine at home and she wished to come home. After coming home she was not ready to take medicine and started refusing food also. She was not allowing my brother also to eat. Her only desire to sell the house and go far from here without any reason. We refused as it was dangerous for my brother and for their child. For last two years she is at Kolkota -Barrakpore. With her baby. In between she comes to my mother in law, or meets our maid tells so anything rubbish, fights, give threats and so on. We have come to know she does not have any house, nor earning properly, not capable of giving education to the baby. Beats her. Mentally and physically tortures the child. Last Monday(21/09/2015) all of sudden she went to my mother in law at Barrackpore residence with the child in 102 degree fever, had not given food for 2-3 days, kept with them saying "I will take her after some time" and ran away. She also said if they will not keep her she will sell the child. My mother in law is 80+ and sick and handicapped stay with a maid so was unable to do NC or General Diary(GD). Again today(27/09/2015) she came and said that she is taking the baby girl who is now 5.3 years old. She had kept her child here to give trouble to everyone. The child is also so much tortured that she was not ready to go with mother. But police whom we called said that according to law they can't deny mother taking a child even though they also knew the mother is very notorious. Often gone to jail for various reason including once she was held for prostitutional offence. Now actually we are seeing the dangerous for child's welfare in dang our in every way like physical, emotional, educational and over all wellbeing and growth. Please guide us as our priority is girl child's safety, her education and her future. If child is deprived of her basic needs in mother's custody and when father is ready and capable of looking after child's wellbeing in every way then in what way he can get custody of child ? This is in very short. Actually we all are suffering due to her imbalanced way of behaviors and can not do anything for the child's well being . Please please help and guide us to save the child's future. And please guide us about how a father can get daughter's custody to protect her from harmfully affecting the possession of her mother? And from harmful objects of socity? My email id is