I’m arguing both sides here in different comments.
You’re right, logically it seems silly at first glance. The basis of nearly all our laws is “If harm to another, then some harm to you.” So if close relatives, a Brother and Sister in this example, were to be each other’s Prom Dates into the late evening, and no one knew they were related, do we actually hear that tree fall in that forest?
Probably not.
So thus they have harmed no one.
So why?
I think because collectively we have an innate understanding of who we are closely tied to by blood we base our laws off the majority rule. At a glance there is nothing wrong with the act. I’ll even argue it happens far more than we know, though also far from being common.
So then it becomes a question of culture. Laws exist as paths that guide a society. Without it the very significant number of people who could do something because they have a different moral code, but don’t because they want to follow the law, would then be free to act without restriction. Besides, as I said in another reply: It is less about others in this case and more about the two doing the act and the potential to bring in a child who should never have had anything to do with it in the first place.
Anyway, I’m interested to hear other arguments. No reason we can’t discuss either way. Though I doubt it’ll bring some great revelation.