Fake friendship

So my fiancés brother is dating this girl we both dislike. She’s really mean to him and they’re honestly awkward to be around, but my fiancé and his brother are close so we suffer through being around her. We used to be friends, don’t get me wrong, but she’s grown to hate me ever since I got engaged (she got drunk one night and admitted she was mad about it) because her bf hasn’t proposed and they’ve been together just as long as we have. Anyways, every time my fiancé hangs out with his brother she’s there too. It’s really hard because I have to put on a nice face even though I do not like her and she’s hurt me a lot in her words and actions. I’ve tried to bring it up with her before but she refuses to listen. Last time I tried it got to the point where we didn’t speak and my fiancé never got to see his brother because she didn’t want to see me. I learned I can’t be honest about how I feel about her for the sake of my fiancé getting to see his brother. How do I keep faking this friendship with someone I know hates me just so he can see his brother? It’s making me unhappy. I don’t know why he won’t leave her when he’s clearly not happy. It’s causing pain for everyone around. Any advice?

Update: she never lets him hangout just the two of them. She doesn’t have friends outside of her boyfriend so she always wants to be there too. She gets mad if she’s not invited. So if it’s not me having to deal with her, my fiancé does.