I've been in the same position before.
My best friend went out with my brother, who was a couple years older then her, and she even had sex with him! WHEN SHE WAS 12! Ugh. Lets just say, I wasn't too happy about that. Whenever she was over my house to hang out with me, she'd hang out with him. And sometimes she would sneak downstairs and go to his room in the middle of the night, thinking I didn't know. I hated it. We got into a lot of arguments about it, and we almost stopped being friends because of it. They're now broken up, but my brother is practically stalking her, and he still wants to have sex with her, when she as a boyfriend. He just doesn't get it....
If I were you, just try to stay out of it. I wish I had.

If they start going out, don't start anything with them, and hope it won't last long. lol.
- L i Z
Sorry if I wasn't much help...