My sister's husband wants to stay with her after he insulted, forced her out of house and beaten her

Hi Everyone, My sister got married in 2018. It was an arranged marriage so there were no family disagreement. The problem started after 3-4 months, he started abusing my sister, beating her sometimes, also complained us that my sister can not do any household work, she is not smart etc etc. We tried several times to solve his problems and console him in many ways but never worked. Sometimes she was afraid to tell anything to us. In the month of september,2019, We brought her home and warned them if this continues then we will not let our daughter go back to her husband's house. They didn't bother to call my family or my sister within first 3 months. When the lockdown started due to Covid, his relatives and sister started calling us and ordered us to let her go and let them live a happy life but the main person(the husband) didn't call my family or my sister a single time. They are continuously abusing us on phone and blaming us for spoiling their brother's future. 2 Weeks ago, He came to our house with friends at 11 pm. He then started shouting in front of our house, misbehaved with my parents and tried to break our door. We filed a GD(General Diary) in nearest police station for social harassment. Today, we got an application from their lawyer that we need to give our daughter back to them within 7 days otherwise they will take further actions. It's not an official notice from court(there is no govt stamp, it's a personal page from lawyer's notepad with lawyer's signature). I just want to know what are the charges we can give to punish him. I don't want to leave my sister to them because I don't know what will happen next. My sister is also in kind of trauma because of what happened. If they want mutual divorce then we also agree for that but whatever my sister was a housewife and she didn't earn and we have spent a good amount of money in the marriage. How to secure her future? How much time it takes to get mutual divorce and what are steps we can take after that kind of letter from their lawyer? As my sister is in trauma and never experienced court culture before, can anyone filed a case on behalf of her? Thank you in advance