If Planning for Divorce what kind of things we need to look

Hi Madam/Sir, I need a advice from you. My sister is facing problems from her husband.He is abusing her daily using unparliamentary language.he is not allowing her to anything to her wish.he will not allow to call to her parents when he is in home.she should be slave for his parents and a kitchen worker. My sister gave birth to 2 baby girls . from the day second baby girl is born he started torching her from third day she discharged from hospital. I will explain one of the incident :::: After 2nd Baby Girl delivery after a gap of 3 days she discharged from hospital immediately after coming to home he is asking her to clean the fridge he is pushing her to sit in front of the fridge to clean . But she is sacred to live alone in the society with 2 baby girls Please suggest me how we can claim children maintenance or property claiming from him what things need to take care i mean any kind of documents we need to save or anything before applying for divorce And Both Husband and My sister stays in United states of america (yet didn't had citizenship form USA) by profession he is Dental doctor .And both the baby girls are USA BORN Thanks