I'd like to say that I think fucking your sister or first cousin is sick and wrong. But on the other hand, I don't have any hot sisters or cousins, so I must remain neutral on the issue.
I do have a half cousin who's pretty hot, though, if that counts. One time I was driving in Wenatchee, and I looked over at the car driving next to me to see if there were any hot girls in it. There was one, so I gave her a "yeah, I'm driving my car right next to yours, and I can see you and I like what I see" look. She shot me a Joey Tribbiani "how you doin?" look (Joey Tribbiani was a character on the television sitcom Friends in case any of you Seinfeld nerds were wondering) and for a split second I felt like I had finally mastered the art of hitting on girls while driving. But then we both did a double-take and realized that we were cousins. Half cousins, to be fair. Plus she's half Mexican, which somehow makes it seem better. Something tells me she was still dtf even after realizing I was her cousin. Lord knows I was. But making a move on your (half) cousin is pretty risky. In fact, that reminds me of another story involving kissing cousins, Wenatchee, coming of age, and Schmidt beer. But I'll save that one for a more suitable time.