Daughter-in-law harrasing old parents

My father is retired person and my mother is homemaker. We are two sisters and one brother. We both sisters are married and staying with our families happily. My brother's wife is teacher in private school. My parents are taking care of their 2years old daugther from the day1. Parents are even taking care of finanical needs of home. She everytime decline to do any work at home. My mother is doing all work at home and taking care of her daugther as well. So manytimes she left home, even in the night by calling her parents. Her parents are supporting her to not do household work and to not give any penny to my parents. Earlier she and my brother use to give some amount of money to my parents to run home. But from last few months my brother's school is not paying well to him, so he was not able to give money. Then she started threatening my parents and brother that if anybody will ask money from her then she will call police. She is taking benefit of my kind parents. whole life they worked hard and now she is insulting them. They are doing everything for themselves and for complete family. She dont take care about her daugther even.She always use to threaten them. I am worried about my innocent parents. she daily do something so that some quarrel happen. My parents are dying day by day becuase of her mental torture. What we should do?