Aren't most incest-related laws in place to add charges to child molesters? I don't see any deterrence value they have because someone who is molesting children is already doing something illegal. In for a penny in for a pound and all that. If two consenting adult relatives want to fuck, who cares? As long as there hasn't been grooming/abuse it has no real problems other than being a societal taboo. Even the chance of birth defects is fairly negligible unless your family has tons of hereditary shit that could be potentially dangerous if two people carrying them all procreate.