How to gentle parent an asshole?

My brother's girlfriend (27) sucks but he's got mommy issues so I gotta be nice to her or I'll won't see him as often cause she'll avoid me and he's her slave. I have to be nice to her but she says horrible things and I feel like it's disrespectful to my values to ignore. Whether it's bullying her mom into drunk driving to get her ice cream at 1am or calling girls sl*ts and wh*res because she's insecure about her body. (not an assumption, she said she was insecure before insulting these women for an hour) I am disappointed I just stayed silent and didn't react at all when this happened last time but I've never stood up for my beliefs before. I don't like that about myself and I want to change and this is a good opportunity to practice.

How do I casually and calmly correct someone without making them hate me? It's mostly for myself and the people around us because I don't think she'll ever change.