It happens, try not to worry too much about it, of all the pornographic stories on the internet, the number of incest stories pretty much dwarves everything else, it's some sort of american obsession - they can't all be doing it, but it does seem to crop up in fantasies an aweful lot.

"familiarity breeds", as the saying goes, and you know your family better than anybody - it actually became quite a problem back in medieval europe under the feudal system when people weren't allowed to travel about to other villages to find mates, for fear another lord would claim them or their offspring - the church finally had to step in and forbid it.

Anyway, to some degree, as screwy as it sounds, fantasizing about it appears to be a pretty common stage of sexual development: probobly a good reason most men seldom ever talk about their masturbation fantasies - you just need to broaden your horizons, get to know some girls, it'll pass.

If you have a genuine ocd, you might try pornography: letting someone else "do the fantasizing for you" might take the load off your imagination.

Thanks for the help, much appreciated.