Cheating with boyfriends brother behind his back

So I did post this before so some might remember. But now I’ve made up my mind. My best friend is cheating with her boyfriend of 4 years with his brother. They all live together in the same house with his parents. It’s wrong and upsetting. I know her boyfriend and he is a really sweet guy who truly loves her and treats her right. When she told me I was surprised for the fact that I never thought she would stoop so low but kinda not surprised because for a couple months when we talk ( we talk and text every day she moved to another state a couple years ago with his family) all she talked about was the brother and what they are doing like playing bored games going out to the movies etc. Her boyfriend works and she and the brother doesn’t so it’s obvious how they had a lot of alone time together. I was thinking she had a little crush or something but would never act on it because it was out of character of the friend I thought I knew.... so one day she text me and says I have something to tell you but you have to promise to never tell anyone. So of course I promised and asked her what does she need to tell me. ( I instantly thought about my suspicions of her having a crush on the brother or maybe they kissed or something) she tells me she had been hooking up with his brother I asked what do you mean hooking up? She say having sex.. I asked how many times she says like 10 ( so I honestly believe if she said 10 it’s probably more) of course I gave her my mind of how wrong that is and she should just break up with her boyfriend because clearly she doesn’t love him.

So last time I told you guys this story you all told me to tell him ( the boyfriend) what she is doing and I just couldn’t out of loyalty to my friend. But now recently they came to visit me and my fiancé and she saw my engagement ring in person for the first time and she kept pressure him about when is she getting her ring. And of course being the sweet loving guy he is he wants to take her out to see rings she likes right away.. ( I know this cheating is still going on with his brother and the whole time we were hanging out I was extremely uncomfortable) so me and my fiancé decided we are going to tell him.. I will probably lose my best and only friend but this whole thing she is doing is to disgusting to watch and keep secret. I feel bad because I will be causing a lot of drama in other people’s life her and her boyfriend, brother and brother, and the whole family that all live together. Thinking about that makes me wonder if I should just keep my mouth closed or should I say something? What would you do? I do have all the proof from multiple text.. I lost all respect for her because I can tell she doesn’t even care about what she is doing. I know her boyfriend will be heartbroken 😞