Aita For Ruining My Best Friend's Wedding

AITA for ruining my best friend's wedding by revealing her secret affair with my boyfriend? I never wanted to cause trouble, but I was pushed to my limit. My best friend Lily and I have been inseparable since college. We shared everything, our hopes, our dreams, and apparently our boyfriends. I was dating Mark for two years, and Lily was dating her boyfriend for the same amount of time. Everything seemed perfect until Lily and I started noticing strange things. Lily's boyfriend, Jake, was acting secretive around her lately. He was distant, avoiding her calls, and often disappeared for hours. I tried to ignore it, but I felt something was wrong. One night, I accidentally saw a message on Mark's phone from Lily. It was a heartfelt confession about how she felt about him and how she wanted to be with him. My heart dropped. I could not believe Lily had been sneaking around with my boyfriend behind my back. I confronted Mark the next day, and he broke down and admitted they had been seeing each other for months. I was devastated. I felt betrayed by both of them. I decided I had to tell Lily what I knew. I planned to tell her at her wedding rehearsal dinner. I thought she deserved to know before she married Jake. I took her aside and told her everything. Her face turned white. She looked like she had been punched in the stomach. She started crying and begging me to stay silent. I hesitated, but I knew I could not lie. Suddenly, Lily's sister burst into the room and yelled that she knew the truth all along. She had seen Lily and Mark together at a hotel a week ago. She had been planning to tell Lily herself. The room erupted in chaos. Lily ran out crying, and her wedding plans were ruined. Later that night, I received a message from Lily's sister saying I was a terrible friend for exposing her secrets. She said I should have stayed silent and let her marry Jake. I feel like I did the right thing, but I also wonder if I crossed a line. Did I ruin her life forever or save her from a terrible mistake? What do you think?