My brother(18)(M) boyfriend(18)(M) is being unfaithful. How do I confront my brother about it?

I have a younger brother(18)(M) who has recently started dating a guy(18)(M). They've been dating for about 6 months. This is my brother's first ever boyfriend and he's really, really in love w him. They go to the same school. <br><br>About 3 weeks ago I was out for a run in my neighborhood and I noticed a couple kissing good night. It was the guy who was kissing a girl good night. I got closer and noticed that it was my brother's bf. I finally got closer and I saw his face, and it was him and his wrists had those little scratch marks my brother gave him. He had plans w my brother that night but it never happened since I saw him kissing a girl. <br><br>The following day, I saw him at his house next door near the mail box and he was flirting w the mail-man. He went home and 2 hours later, I saw him sucking some guys dick in his car. At this point I should mention that the girl who was kissing good night was a different person than the one in the car. <br><br>This was a week ago and since I moved out of my parents house and into another house in the neighborhood close by, I haven't seen him cheat again but the incident really shocked me and my husband. It's unlike me to snoop but I also feel like I have to let my brother know. <br><br>He's 18 and I feel like an older sister, even though I'm his brother, it's my job to make sure he's ok and I'm just really worried he'll get hurt. But my brother and his boyfriend are really happy and I don't want to ruin his happiness. I feel like I'm catching him and my husband cheating and it doesn't feel good. <br><br>What's the best advice in this situation?