Jailing them is far more wrong. I don't think that laws are necessary or prudent for every human behavior, favored or unfavored. Quite the opposite. I think society would be better served by having less laws. But that does not mean that I think morality is unnecessary or imprudent: Quite the opposite. I would submit that the most free society is one with strong common morality and weak laws.

In the special case of sibling incest, it would have a strong societal prohibition and no laws in that regard. Why should society frown on incest? For the obvious reason that NO birth control method short of abstinence or sterilization is 100% certain. Progeny from siblings is a near guarantee of a permanent burden on society. Pursuit of personal sexual gratification at the potential long-term expense of society (not to mention the gene pool) is narcissistic.

B