Yeah, cos if there was any contact between the two, it'd be incest!
Lol reminded me of a conversation I had with a mate of mine many years ago, I mentioned that we were visiting my wifes (then girlfriend) sister in Brighton. He commented that Brighton was the gay capital of Europe, I said thats why she lived there as she was gay.
He couldn't get his head around the fact that I wouldn't get her and my girlfriend together, all he kept saying was " BUT THEY ARE SISTERS!!!"
The only way I could get him to shut up was by saying that I really thought his sister was hot, he obviously liked my girlfriend, so did he want to bring his sister along and join my girlfriend and I for a bit of all in action, I nearly got a fat lip though when I asked if she was shaved or not.