Mother in laws

My mother in law is horrible. She bothers me and my man, always complaining about something. She tell her son he is a bad son. She is just depressed and needs to be on medication. How do you handle a mother in law you don't get along with? My engagement ring is my finance's grandmothers and it was given to us by his aunt, his mother had the nerve to ask her son if he asked for the ring b/c she doesn't like his aunt. We didn't even know this ring was even around.

I need lots of advice!