Is the incest taboo is natural (genetic/instinctive) or learned behavior. There apparently is empirical evidence for both sides, which I think means it must be some sort of mix. Anthropologists have found that the institution of marriage, and rules concerning appropriate and inappropriate sexual behavior, exist in every society.

[I am no expert but I suspect that there is a dearth of recent studies on taboos, most of the major works by people like Meade, Boas, Geertz, Sapir...all were kinda mid 20th Century. So...]

Individuals that are closely related typically have harmful genetic mutations. I am sure humanity learned that and most animals instinctively seem to know that, usually. It's kind of like a genetic failsafe. You don't want individuals that are too closely related as you won't get enough genetic variation. You want variation so new genes can evolve and eventually have a beneficial mutation.

In this specific case, I'd say our ethics and laws on the issue stem from natural, biological laws.