Generally incest is a social construct, aside from power dynamics, even horizontal (e.g. siblings biological, step, whatever) incest suggests some dysfunction in the family dynamic when growing up, because we know things like the Westermarck effect make incest less likely.

But there's edge cases. Like people who were adopted and never met in childhood, like it's not really incest, though there's obviously genetic concerns which makes it advisable for them to not have kids (with each other).