As with any situation involving consenting adults, I don't really care.
Does this include literal incest? (Like full sibling, or even parental incest if the child is over the consent age?) Or polygamy, for that matter? Polygamy would open up such a can of legal worms: wills would be so much more complicated.
Fair questions. I don't think there should be criminal penalties for consensual incest, definitely free counseling though. Polygamy appears to me to impede the rights of most of the people involved, and leads to highly dysfunctional social hierarchies, and though I can entertain an argument in its favor from the libertarian perspective, I cannot embrace it because of the effects of polygyny when it thrives as an integral feature of isolated cultures.