If I we're you b/f or sister, this is what I'd tell you:
1. We all look, but touching is a no, no!
2. Keep your friends close, and your enemies closer. If they are "just friends" and he loves you, he won't have a problem with you being friends with her.
3. If you have SOLID PROOF they are having any type of sexual talk or (god forbid) contact, you toss him to the curb like the trash he is.
4. I hope that he never does this to you, but no matter what happens in your life, good or bad, you always need to find something positive in what did happen.
Example: My ex's b/f stirred the pot between my ex and I and my ex's ugly, abusive side came out. His b/f was lying and I proved the idiot was lying, but the ex ALWAYS took his b/f's side. MY positive: If it we're not for the ex's b/f doing the things he did, I never would have seen my ex for his true colors. While his b/f thought it was funny to cause problems and know that I was being abused from the lies, what he really was doing was a favor for me.
I hope things go well for you! Good luck!