Mother in law throwing things

My fiancè wanted to go see his mom for Mother's Day. I didn't. Nothing against her. She's just never said two words to me. She doesn't talk much at all because she's extremely mentally ill to the point she has to stay in a home for severally mentally ill adults. My fiancè and his mom have the same dad because did dad was SAing her for years and got her pregnant when she was 13, had him at 14 and had 3 other kids with her dad. So him and his 3 siblings all share a father with her. I understand completely she isn't all there mentally but I have never been able to form any relationship with her. She talks rarely and doesn't even acknowledge my existence. On Mother's Day my husband was cooking for us and we were at her tiny little living area. I'm not exactly sure how I set her off but I did and she threw a plate at the wall and flipped the entire kitchen table then went to sit in her bathroom and bang her head against the wall crying. A nurse had to come in and help her calm down and my fiancè basically carried his mom to bed and sat with her talking to her until she went to sleep. I just don't see how I'll ever get a normal mother in law, daughter in law relationship with her.