1. No point continuing relationship with such type of abusive person,
2. Let your sister get rid of her and start her life afresh,
3. Let her collect evidence through audui/video recordings about the said torture on her and lodge a police complaint u/s498A,
4. She can also file a DV case against her husband and inlaws,
5. She is the natural gurdian of her 4 months old baby,
6. Arrange for her going out of the house with her baby after which she can go to any place she wisheds to go,
4. Her husband has to file child custody case to claim custody of his child.