When I was unmarried and in my twenties, my cousin's fiancee blatantly propositioned me a couple of weeks before her wedding to my cousin. She was an attractive woman, but I deflected her advance. And delicately told my own mother about it, just in case my soon-to-be cousin-in-law was embarassed by my rejection enough to claim that I made an advance on her. My mom and I tacitly agreed not to say anything to anyone else. And when I later married, I told my wife so that she understood why I never got too close to the woman.
What drove my rejection was simply this: It would have been nice to fuck her silly. But where the hell was that gonna go from there? Did I expect that I'd be screwing my cousin's wife for years thereafter? Nah, no good can come from those kinds of family secrets.