1. First of all your sister has to decide about terminating her matrimonial relationship with her husband since after taking harsh legal course of action it will be difficult to restore normalcy in the relationship,
2. If she decides to to hit back at the cost of the relationship the first step she shoud take is to start collecting all evidence of cruelty on her (for dowry or otherwise),
3. Once fullproof evidence in the form of audio/video recordings etc. are collected, lodge a police complaint u/s498A of Cr.P.C. against her husband and in laws,
4. File DV case against them praying for maintenance and seperate residential accomodation for her and her children,
5. Before filing the above cases, there should be adequate litigation planning and preparation for her leaving matrimonial house to settle in her parental house or elsewhere. It should not be done with preplan and not while in a crisis,
6. After those cases are filed also prepare to contest case expected to be filed by her husband for child custody since her one son is 10 years old for which his custody may be asked by his father. Custody of children above 5 years are decided by the Court based on welfare of the child,
7. Audio/Video recordings are valid evidence which are well accepted by the Court.