What to do about my cousin?

Hi all, im stuck in a situation and ill try brief it as much as i can, but im seriously stuck on what to do, so here goes.... my 22 year old cousin who im also best friends with has changed alot. She started hanging out with a guy called Peter from her work. Hes in his 40s n married. The whole family were scared of his intentions at first as we didn't see this as normal. Then she starts hanging around another work mate (trevor) who is also in his 40s, married and has kids. I had talked to my cousin many times to make sure she wasn't getting herself into trouble with married men, and she swore nothing was going on. In march this year she confessed to me she slept with Trevor's son Chris. Ok great someone her age cool, but hold on, chris had a girlfriend, and he doesn't want anything from my cousin but sex and my cousin said ok. This is wrong in my opinion. However her and i hadn't talked for a while so long story short she Texted me last night to say she ddoesn'tinvite me out because its hard now i have a baby as i always have to bring my baby with me. This made me extremely mad and hurt! The messages ended up a bit nasty as i tried to understand why ive been pushed aside for these men. Not only has she been arguing with me about this situation but also her parents, brother, bbest friendfrom school and even her grandmother. So i ended up talking to my cousins best friend from school and she filled me in on the stuff i didnt know. My cousin and peter have been doing sexual activities (not sex) since last year, she is inlove with him and wants to have sex with him. He wants to divorce his wife for my cousin, but my cousin is scared because of how the rest of the family would react. also he stays at her house friday nights, keep in mind he is still married, but wife thinks hes working. They have also been called into the bosses office at work (nursing home) and nearly lost their jobs as residents and their families are complaining about their behaviour. Her best friend and i had a long chat last night and we feel something should be said as she is getting herself in alot of trouble. i was thinking maybe just warn her brother with whats happening? Can someone help, what would you do if it was your cousin. Would you step up before she loses everything or let her learn her own lesson?