But how would you propose the community deal with it? Institute something like the Kirk Session? Also, I would disagree with you that the law has not helped here. It has probably prevented further incest behavior and children being produced from said relationship.
This is a difficult situation I think for libertarians, but I am under the belief it should remain illegial because: children, with possible genetic defects, would likely be born of such a relationship, and as I am opposed to abortion, I do not see many other options in protecting the innocent life- that is, the child that would be produced. Thus, I don't think this would be a case of merely two adults and no others involved.
In a free society, the community has myriad means other than governmental force at it's disposal, to deal with things it considers unacceptable.
For example, the individuals in a community may decide that they do not wish to trade with Mr. Incest.... or sell him goods. Or allow him access to stores, clubs, social institutions, etc. The may decide to ostracize him. Shun him. Protest against his actions with signs, editorials, and demonstrations.
They may do all of these things morally and effectively.
What they may NOT do morally, is initiate force against him. They only morally legitimate use of force is in defense of rights. Hence the use of force must be withheld unless he violates the rights of another.