AITAH for finally giving my best friend the proof she needed that my sister slept with her boyfriend the year prior.

Okay so to set the scene, when this happened I barely knew the girl who came to be my best friend. I met her through my boyfriend, and it was pretty obvious to me that her boyfriend was SCUM when the first thing he said upon meeting me was something along the lines of asking if I was exclusive to him or “for the gang”. Smh. So flash a few weeks I don’t really know this girl. One night I’m hanging out with my sister and my boyfriend and his friend , we’ll call him … Vick for the story… Vick is there too. At some point in the night, I black out (we all were drinking) and I’m left in the car with a tapestry on me. When I wake up the next morning my boyfriend is next to me sleeping in the drivers side of the car and I check my phone. I see messages from my sister talking about the size of Vicks … you know. I was so hungover I really didn’t care to respond and we never really talked about it. So I’ve been with my boyfriend about 3 months in this part of the story.. as he is best friends with Vick, I am starting to be around his girlfriend , who we’ll call… Sharon. So over time I’m getting to know Sharon, we’re hanging out all the time. I’ve mostly forgotten about what happened that drunken night. Out of nowhere, Sharon starts expressing to me when we are alone that she is suspicious that Vick slept with, none other than my sister, who we’ll call, Kena. I immediately text Kena, kind of frantic asking what I am supposed to say, if she’s okay with me telling her now because so much time has passed. She insists that I must keep her secret, and this isn’t the first time I’ve kept her secrets of her cheating on her male partners out of fear, because my sister resorts to physical violence and I was living with her at the time. I didn’t feel it was my place to say anything and I didn’t want to jeopardize our relationship, my living arrangements, etc. Weeks keep going by of Sharon saying ooo I know he did it , I keep finding suspicious things in his phone and blah blah blah if only I had proof. Well after 9 months of listening to her go on about wanting this proof and she would leave him, I finally asked her, like would you even really leave him though? He’s cheated before and you forgave him. Would you even really want that proof at this point when you’ve been committed to staying ? She insisted that she would leave him, never speak to him again, etc. I struggled with feeling like a bad friend for lying, and a bad sister for wanting to tell the truth about what happened. She told me that when she asked Kena about this, that no it wasn’t Vick who she slept with , but another friend of my boyfriends and Vicks, who will remain nameless for this story. When Sharon told me this, I was confused because that friend had been in jail during the time that the incident occurred. I finally gave up, I was tired of being in the middle of the drama and I just sent her a screenshot of my sister bragging about the size of Vicks manhood. What do you know, she did NOT leave him. She was furious with me for not telling her. My sister was furious that I told on her, because her baby daddy she was messing around with found out from Sharon, and it caused turmoil in Kenas love life. I personally feel that, none of this wouldn’t have happened, if my sister Kena, would’ve remained loyal to her man, and not slept with Vick. She blamed me for the downfall of her life, because apparently she had more situations going on than I knew and she had no business sleeping with Vick herself. It has been a few years since this has happened, and I reached out to my sister and apologized for my part in things. I just asked her to acknowledge the weird position she put me in, to lie to my friends, and be in the mix of cheating when that is not a lifestyle I will live or condone. I only held my silence as long as I did because that was my sister and this girl had shown so many times , she wasn’t going to leave him for cheating. I was at a crossroads. I did have a friendship with Sharon eventually again, for a time, but my sister never came around. She has refused to let me be apart of my two nephews lives anymore, she keeps making threats to physically harm me, and she continues to live her life of immortality. I am a Godly woman, and yes I’m the younger sister (24 and Jena is 29) but sometimes I feel like I am the big sister , trying to instill morals into my lost sister. AITAH for for finally giving my best friend the proof she needed that my sister slept with her boyfriend the year prior.