How to file a defamation case against wife harassing her mother in law

My wife filed a divorce petition at her matrimonial home & before that she sent legal notices at my and my mother's workplace. She tried to defame my mother in all the notices that she has affair with someone. Once by mistake one porn video came to her mobile and made base of that thing without knowing the fact. After all the notices health of my mother has also been affected and hurted her a lot. We replied all her notices. Now I wanted to file a defamation case against my wife to harass my mother. Please give your valuable suggestions.