1. What is the nature of torture inflicted on her? Has she been subjected to physical torture also in addition to mental torture? Has the torture been inflicted with respect to dowry?
2. Depending on the answers to the questions formulated above your sister can bring the required charges against her husband and in-laws. She has abundant remedies under the legal framework to punish the wrongdoers. If she has been tortured for dowry then she may file a criminal complaint for dowry harassment against her husband and in-laws whereupon they may be arrested and punished by the court. As and when they apply for bail she can oppose it through her lawyer. Your sister can also file a case for domestic violence against her husband and in-laws to put an end to her victimization. She can also seek the right to residence at the expenditure of her husband. In addition thereto, she can also seek financial support for herself and her children from her husband.
3. Is your sister residing with her husband? If yes, then in order to claim the custody of both her children she is required to file a case for child custody.
4. You cannot file any case against your brother-in-law and his parents for the treatment they have meted out to your sister. The right to institute legal proceedings for violation of your sister's rights is vested in your sister alone. However, if any wrong has been committed on you by them, you may take out the required legal proceedings against them.
5. Your sister would have to produce proof in the form of audio/video recordings, witnesses or medical reports (if she was subjected to physical torture) to nail them in the court. Voice recording is a valid proof if the conversation recorded inevitably shows that an offence has been committed. .
6. If your sister's mother-in-law is defaming her then she can file a case for defamation against her.