Awful Mother in law...

This is going to be long so thank you ahead of time for reading. Does anyone have a awful mother in law who tries to destroy your and your SO life? If so how do you deal with it. My fiancées mother is always finding something to complain about and goes as far as calling my parents. Have in mind I live four hours away and in a different state than they are. Which really bothers them being my family is very close. This last time she told my grandmother that I did not need to be with her son bc he was going to amount to nothing that all he does is lie and cause heartache which is a lie from the pits of hell bc all he has ever done is pamper and worship the ground I walk on . Yes we have our fights but they always end up in laughter. Well my grandmother decides to come up with my mother and see what the whole fuss is about. My mother in law acted like a two year old when my mother pulled up in the driveway and she ran off saying she was going to call the cops well she went off on my mother and I stepped in she told me to leave that I was a fat whore that was going to end up like her son good for nothing. Then she called the cops. When they got here she ran inside and hid so we had to talk to them to our advantage he was very understanding and told us that my fiancées mother needed help and good luck being married to a man with a psychotic mother. If you have any advice on how to deal with this please tell me and thank you ladies for taking your time to read this.