Should I say something to my future MIL

My fiancé and I have been together for 4 years since High school and I've met her mother and I hate her. I honestly hate her whole family. Her mother and her 3 sisters try to put her down any chance they get and she takes it. They've made her cry so many times. They make fun of her weight and her hair and her face. We had a miscarriage a year ago and her mom said "Guess thats another thing you need to fix about your body" and her mom and her bitch sisters started laughing. She told me not to say anything and it will just make things worse. They were going to look at wedding dresses and I didn't want to go but I know if I didn't she wouldn't have anyone to tell her she looks beautiful. She was trying on a pink one and her mom laughed and told her she looked like someone's stood up prom date. My fiancé has always told me to never say anything and hold my tongue and I wanted to say something but I could see her giving me a look. Her mom and her sisters bullied her the entire time to the point she went to cry outside and I followed her to give her a hug. The lady at the place asked if she wanted us to make them leave so she could try on dresses without being bullied but she just wiped her tears and said "Its okay, I'm used to it." We ended up leaving. I want to talk to her mom and give her my two cents but I know it will get back to her and she will be upset because it will cause drama but if she's not gonna stand up to her family somebody needs to. Should I or just respect what she wants and let it be.