Anyone have any advice for those who own the title of the "third leg"?

I'm actually at a convention right now where this is happening. It's me, two friends who are dating, and the female friend's brother. Every time they cuddle or get close or show affection towards each other, my mind just shuts down emotionally and socially. I either avoid all eye contact and clam up, or I simply leave the room (we got a fancy room, so the living area is separate from the bedroom).

There are more complicated circumstances behind this, but those aren't necessary for this thread. I just want to know how others deal with being the odd one out.