Harassment by father's brother

Hello, I am living in USA since last 9 years and my sister lives in Newzealand. We don't have any brothers. My father was having an elder brother who was deceased in 2010, one younger brother and one younger sister. My dad's younger brother is very greedy person. He is claiming all the properties of my father's, my dad's elder brother's and my grandmom's property should belong to him as he is having a son and my father doesn't. My uncle threatened my father in various way to obtain the property with no success. So now he is going to me and my sister's in-laws houses and telling them all the bad things and lies like me and my sister were having affairs before marriage but me and my sister we never involved in any affairs , ever. Now my question is me and my sister want to file cases against him, so under what sections we can do so? Because it's unbearable to listen his allegations. He is defaming us. We are unable to tolerate his words. These allegations causing harm to our dignity and causing trouble with our in-laws. Please kindly help me and my sister. I really appreciate your help.