AITA for staying silent over my sister's cheating?

We're both in our early 20s. I was the best man at my sister's (24f) wedding last year and my girlfriend (21f) was a bridesmaid. During the reception, I noticed my sister's behaviour was a little off. She spent most of the night on her phone and whenever I tried to hug her or give her a kiss, she seemed uncomfortable. I assumed the worst and followed her into the bathroom, where I caught her giving oral sex to a guy I recognised as one of her old flames.<br><br>I left without saying anything and went back to the party. My girlfriend and I left early and went to my friend's house to get high. He asked me if everything was okay and I told him what happened. He said that I should tell her husband, but I said it was none of my business. He seemed a bit weirded out by my response.<br><br>Now it's a year later and my sister is getting a divorce. She told me and mom that her husband cheated on her, but she seems really happy about it. She was grinning and wanted to go out to the bar to celebrate her newfound freedom. I was happy for her, but my girlfriend said that I should tell her that her husband didn't cheat. I said it's none of my business and if my sister wants to keep some things private, that's fine. She got mad at me and said that I'm a horrible brother and I'm letting my sister live a lie.<br><br>AITA for not telling anyone about what I saw?