The problem with incest (apart from the genetics issues) is power imbalance and abuse. Fathers have authority and power over their daughters until they're 18 years of age (by law), would it be considered child abuse if a father primed his daughter to have an active sexual relationship with him by her late teens (age of consent)? Seducing their child all the way into adulthood... it's unethical and immoral, should it be illegal? Should governmental resources be spent preventing/prosecuting these cases? I dont know. Apart from that, I think it's easy for an authority figure to coerce a young adult into sexual acts, given the right circumstances. And then, at the end of the spectrum, there's downright sexual abuse (the duggar family comes to mind).
But ultimately, I think it's wrong for the government to walk up to two consenting adults and imprison them because of sexual acts in private. If cousins, siblings, etc. want to engage in sexual acts, whatever. When it comes to parents, uncles/aunts... I dont know, that's a gray area.