I told my sister

So my sister in law is getting married in 2023 she has 3 kids a 5 year old, a soon to be 3 year old and a 6 month old baby. My boyfriends bestfriend is my sisters boyfriend except my sister and him been on and off since she was 16 years old so its been a few years. Anyways , my sister in law and I would go to clubs and she had asked my bfs bestfriend to be our designated driver since we didnt want to drive drunk. When we got to the club everything was cool. They ended up leaving me in the club to go to the car I was drunk so i didnt really care .. i called my boyfriend and he was angry and yelling at both of them for leaving me. After that, we went to the club a few more times. I had noticed that my sister in law & my boyfriends bestfriend has gotten extremely close. My boyfriends bestfriend and my sister started talking again and eventually my sister slept with him. When she did , my sister in law told me she had to tell me something , so I was like okay what is it ? She told me not to tell anyone. So she told me that my sisters lil boyfriend (also my boyfriends bestfriend) has been coming on to her and shes been ignoring him and telling him shes about to get married . She told me that he said to her that he regretted sleeping with my sister. I know i told her i wouldnt say anything but i had lied. That is my sister , i dont want my sister looking dumb or getting played by anyone. When my sister ended up finding out, he had admitted that my sister in law & my boyfriends bestfriend both been seeing eachother in secrecy and it was a two way street.. after that he felt guilty and admitted everything to my boyfriend face to face. Now my boyfriend’s family is looking at me bad saying Im fake for telling my sister something when my sister in law had confided in me. I dont feel as though im wrong because at the end of the day if I tell her that I’m cheating on her brother or making him look stupid she would tell him too. His mother told me that I was wrong everyone has & They have made me feel uncomfortable in family functions.. please comment your opinions and thoughts thank you