Good point, for it greatly "complicates" family relationships, when that weird stuff goes on.

Hopefully, the vagina you came out of, is still for a guy's father to fuck, unless the guy's parents have remarried.

But sometimes people get too carried away with how they define "incest." Isn't the main idea of "incest" to avoid "inbreeding," the passing on of problematic genetic traits? Don't many states still allow for cousins to marry? I think that cousins may be distant enough, to not have the inbreeding problems. The children of Adam & Eve could marry each other, as there were no other humans around back then, to alternatively choose from, and because they were so close to the "orginal" God's creation, that there wasn't yet time for so much genetic damage to occur from generation to generation.