What legal action to be taken in case of domestic violence

My sister suffered of soooo many years of domestic violence as her husband use to abuse her with bad words and put her down often. Took all the household loans on her name and didn’t provide for their kid . Who is brought up by grand parents. He left my sister without notice and is with other girl. He is not communication with any of us but still call and corner my sister to give mutual divorce. We said without discussing with elders how can we conclude. After hearing that he didn’t turned up for years and keeping his whereabouts secret. It’s been two years and few months of complete separation. Even now he is not bothered to meet and decide. He wants my sister just to sign the mutual divorce. My sister is suffering... staying with our parents and working hard to provide her 9 years old kid . But he is staying with that girl illegally. We now decided it’s of no use in waiting for discussion. How long it’s going to take if we oppose wrong entry in the mutual divorce