Regarding my sister marriage issue

My sister are 22 years old & got married her boyfriend on Nov 2020 without our family members permission.. We let her to live with her boyfriend.. But her boyfriend & his mother gave mental torture to her. he often beat her.. She can't tolerate the torture given there so called me to take her back to home... So i went there but her husband refused to send her with us.. She came with us without her husband permission and filed a case against him for his harassment .. Police called him for investigation.. During investigation he said that he won't beaten my sister & my family members only canvassing her to came back home.. We doesn't have any evidence to prove his harassment against my sister.. He also gave false statement against our family members & says that she take her all certificates while she left his home.. But we doesn't took any of her certificates from there.. She doesn't want to live with him anymore but he want to live.. How can we get certificate back & divorce from him?