I caught my 13yo brother having and I don't know what to do!?

So I was in my room playing black ops (I'm 17 btw) and I have a gf, but we're serious about things. We've been together for 4 years and never mentioned but about a week ago we tried it out because you know... we're that age and it's legal. And it was fun and all but we used a condom and made sure it was fun for both of us. But anyways, I was texting my girlfriend and I noticed my brother wasn't talking to his girlfriend because I'm across the hall and would be able to at least hear mumbling. So I noticed that and I KNOCKED and said "Are you ok?" No reply. So I opened the door, only to see him on the floor with his girlfriend on top of him. I knew what they were doing because neither of them had any clothes on. I was so embarred I closed the door as quietly as possible and went into my room. I knew I should've said something... but I didn't. I closed my door and pulled my blankets over my head trying to get the image out of my head. But I couldn't. And now I don't know wether to tell my mom, or to say something to my bro. I mean, he's 13! What should I do? And don't say "you're stupid tell your mom and beat the sh*t out of your bro because he's too young". He's also a human being. I'm not going to beat the sh*t out of him so be reasonable. Any stupid answers will be removed.