Hey, hey, if you're a dude and you get it on with your sister or vice versa and she gets pregnant, won't the kid have much greater chances of having birth defects? Not to mention, the whole Oedipal issues that the kid will face if he/she does make it past a year. I believe this is why incest is frowned upon in Western culture (and most others that I know of) at least, and that's why I would say that even with a third cousin it would be kinda weird but probably ok. With a cousin or second cousin it would be abnormal, to ME at least.