If you live in rural Alabama I bet it's pretty common. And maybe in the white house.

Incest is wrong... socially, morally... and biologically. That there are such prevalent incest taboos; including animals other than humans btw, points out the negative issues w/ this practice. To restate, incest is illegal for several good reasons; for reproductive reasons as too much (close) inbreeding expresses undesirable genetic traits. A celebrated example is the incidence of hemophilia in some European royalty lines.

Incest is amoral. Perpetrators are immoral. Repetitive inbreeding brings defects to the forefront (biology majors?). However incest is not about breeding. Like rape, it's about power. If we had a #metoo movement for incest victims, the numbers (and identity of perpetrators) would sicken the moral population.

First of all: Ewwww, yuck! I would personally have difficulty in thinking about any of my siblings that way.
Secondly: since it is already frowned upon, children of incestuous relationships will be ostracized for having sibling parents. As far as making incest commonplace and legal, if we lived in a world like that, (like Game of Thrones) I still think that the risk is too high for the children. Especially if those children also continue to procreate within the family.

As I've said on a similar topic, kids deserve to be raised an environment without sexual grooming behaviour from their parents and older siblings. The real issue with incest isn't so much genetic diversity, it's more the idea of a mother or father raising their child to believe that it's expected and normal for them to have sex with their parents as soon as they're old enough.

This is something that should be considered when incest is prosecuted. I don't think it should be, unless there's evidence that one party is a victim of the other and either they haven't consented (in which case, it's rape/sexual assault anyway) or they have been brainwashed into consenting. This clearly doesn't apply when two people meet, begin a relationship, and later discover that they're related. The only real argument against that is the genetic diversity one, and if that's as easily debunked as you say it is, then perhaps there's no issue at all.

Having sex, raising a child and having a biological offspring are very different things. One need not have to raise their own biological offspring. In a rational world, offsprings will be planned so as to have best genetic outcome. This would not just avoid the risk posed by incest but would also biologically improve the species. One may have sex with their siblings and raise these planned children with them without any biological risk.