It is morally wrong. 1, the products of incest are prone to genetic disorders. 2, incest is never consensual because one person is in a position of power over the other and it can tear families apart when the relationship end. 3. If they are not blood relatives or gay then the family dynamic will still be ruined, and familial bonds will eventually be severed when one of decides to end the sexual relationship. There's a reason every language in every culture has a swear and a slur associated with incest between close family members and it's not because it's a societal taboo. It's because it's morally wrong. It is uncivilized and equivalent in moral harm to slavery.
If the folk involved never knew they were related until after their sexual relationship started then the only correct course of action is to end the sexual aspect of the relationship and be deeply disturbed that it happened.
The only reason incest porn is currently popular is because degeneracy with a built in racy plot line, and more extreme porn sells better than other types of porn. It's the morally wrong aspect that people watch it for.