Protecting Child Rights

Sir/Madam, I have a -1- year old niece staying with us (me, my mother and my sister) due to troublesome environment at her paternal grandmother's house. Mother of the child was harassed by in laws with father being a spectator, her which led to her and my niece staying with us. Now as days are passing my sister is not sure on what to do about the relationship and father is exerting his right over the child and constantly pestering her to give away the child to him. Father does not earn anything and all his means are taken care of by his parents. Nor is the father or his family well educated. In laws are apt in using foul language. My sister has a job and is earning decent to take care of the child. Out of fear and mental torture at times with likelihood of them harming me and my mother, my sister fears keeping the child. We are worried for the child and do not want her to suffer at any cost. Can someone tell me what should I do to protect my niece's rights and save her from going back to that abusive environment.