Partners mum is a monster in law 😥

So my partners mum is an absolute nightmare, she acted all nice the first time I met her then a few months later like my best friend we became really close then she turned on me and accused me of sleeping with her husband because I had a shower while she wasn't home! He was asleep Coz he does night shift, then my partner told her off because he knows I would never do that and she turns on all of his girlfriends anyway we got over that I forgave her and we moved on then she would constantly pick little things at me and try to act like my best friend again and because she was my partners mum I let her back in & last night she started an argument with me for no reason saying I should be on the pill because I'm a bad mum and not good for her son (when she gives her son drugs and let's him take drugs behind my back doesn't make her 16 year old daughter take her birth control properly, doesn't even send her 14&16 year old girls to school! And helps them sneak out behind their dads back to sleep around)