Friends husband cheating with my brother

I'm so conflicted on what to do because while she isn't a nice person.... She's still my friend. I've been pretty much raising my brother since he was 5. We have a 10 year age gap. When I moved out at 18, he came with me. He's 19 now and gay. I've been friends with this girl on and off for years. She can be very crazy. She's violent, and very vindictive. She got mad at me and drugged me once to get me to cheat on my then boyfriend. I know some might not understand why I forgave her but I did..... She had also said homophobic slurs to my brother(he's gay). She was 27 when she met her husband... Who was 16... He's 20. I know she's abusive and abuses him and a lot of people are gonna ask why I'm still friends with her..... She made him drop out to be with her. He ended up getting his GED. He also joined the army. He lives on base and she's at home. She has never treated him right and manipulated him since his teens... But he still married her when he turned 18...

Anyway I was in my brothers room because he borrowed my old college notes. I saw on his desk he had been writing letters to my friends husband. It was very.... Romantic ... I snooped more... I shouldn't have but I did. There were like 20 letters between them and some were so.... RAUNCHY! How sexual it was... Ew... This seems to have been going on for a while. Idk what to do... If I tell I friend she WILL hurt my brother and possibly me...

Edit: I know she may not seem like the best friend. We've just have a VERY long history and I don't think she was trying to get me raped. I think she wss trying to make me so far tripped out I would want to have sex with someone. Not like pass out drugged.

Edit: I'm not being abused by her. I've been abused before by my parents. So was my brother. She never did what my parents did to me. I am missing teeth because of them. When I was 18 she let me sleep stay at.her house for a week while I figured stuff out..