Domestic abuse and child separation

Good afternoon team, My sister involved in extra marital relationship with a tenant In her home. She used to send all vilgar videos and pictures. It has been notified by their in laws. But then she accepted her mistake and they forgiven her, but then her In law who is very obstinate atrted abusing her and house arrested her. as a result in which my sister committed suicide which is unsuccessful. She established relation due to the fact that her husband was mentally ill. Which she came to know after all these issues. Her in laws after her suicide attempt made my sister to stay In my mother home. She accepted the mistake but they are anusing her and tried to house arest her and they even beat her during her stay even after accepting the mistake. They are not accepting the fact that his son was mentally ill and they hide this issue till date after 8 years of marriage. My sister child is with her in laws they are not allowing to take with her and they even blackmailing her they they will go to any extent to keep their grand daughter with them. She wants her child with her. she can live independently by taking care of the kid. She did the mistake because of her husband. We are a middle class people. We don't have any support from anyone. They betrayed our sister. We accept the mistake what she has done. However, they are not allowing her to settle In the home and black mailing her. They are rich people. We need some help now. It's a request