I don't know what to say. But I have a best friend, known her since middle school, we hang out all the time and she's been staying over at my house. I still live with my mom and little brother (16). It's only been a few days, i came home from work and went to lay down and i saw them. They were having sex on MY BED. Her and my brother.

Why would she do this? He's not even out of school yet. He's not even of age yet, either. That's not the worst part, She has a boyfriend, he has a girlfriend. So they're both cheating. I'm at a complete loss and have no idea what to do. I've never even see them speak to each other, she mostly would just hang out in my room and watch my tv or something. It just seems so sudden, they went from being barely aquantances to sleeping to together. I don't know what happened or why they'd do this.