I know they might use condoms, but let's say they're tested and they don't. Even if they DO, I'm squicked out by the whole "double-dipping" concept, and even though I'm realistic and understand that it's not for me to make this decision for other people and their decisions don't affect me, I think EVERYONE should be squicked by it. I'm an only-child, but I know if I had a brother OR sister I wouldn't want to share a boyfriend OR girlfriend with either. I also wouldn't date someone who was dating one of my PARENTS. Any first-degree relatives should not intermingle this way. There are plenty of other people in the world.
I'm not even sure if "squick" is in the dictionary; I even hate using that word, but it fits this description. I have consulted my solar plexus about this and refuse to let my brain rationalize away something that I know to be true.