Yeah, because he was drunk he didn't know what he was doing. Come on. You don't do things drunk that you wouldn't do etc, etc, etc. And I seriously doubt that she was so out of it that she didn't realize what was happening, either.
I get the feeling that you posted the thread because you "noticed" her and then some. She'll sleep with a relative in a pinch, so. apparently she's easy. Go for it. That's what you want to hear, isn't it? That, or just don't bring it up. Their issue, not yours.
I had a friend/associate in high school who had sex with his sister on multiple occassions. It was Summer the first time it happened, and we all played him in baseball right afterward. When he came to bat the infield and outfield changed "incest is best" to rattle him. It was fairly cruel.
Then he got her pregnant.
Ah, young love.