Sexual harassment by father-in-law

I have a sister who led peaceful life, but her father-in-law tried to abuse her sexually. Anyhow she denied all attempts just because of husband's support. But her husband has an illegal affair with another woman. So my sister applied for divorce. She filed complaint on her father-in-law, the defending lawyer from their side said he is old so he can't appear in the court, he is a womanizer. Many of my relatives in his hometown saw him lead a usual life.

We would like to get an arrest warrant on him? What to do ? How to do ? Because my sister went under so much mental torture.