My father is getting tortured by my sister in law

I have lost my brother in the year 2018 due to cardiac arrest. At that time he has two daughters (6 yrs and 3 yrs). My mother was also expired before my brother marriage.At that time my father requested my sister in law to stay so that the girls can continue their study and they are the only memories of my brother and my father told her I will take care of their all expenditure till their marriage. She got ready but after three months suddenly her behavior was changed. She filed a complaint regarding my father for rape and we are two sisters she also told that we two sisters are also torturing her. We were completely shocked we tried to ask her what happened then she said I don't want to talk anyone and I want to stay in this home and you don't even try to talk with me and my daughters. My father is 72 yrs old he felt so ashamed that he decided to stay away from home now from last one year he is just passing his time and waiting for his death. We sisters are really heartbroken we don't have anything in our hand because she told us if you try to do something I will again file a complaint of rape. And nothing can be worst than this, a aged person getting blamed for rape. Please help what should we do. Thanks