I found a study that said that almost 20% of people in it had reported having some sort of experience in incest, and given that not all people would be willing to admit to it, if that number might be smaller than the true number, especially since it's not representative of the global population, and that lots of people seem to like it in fantasy which might mean it could possibly translate to a more real-life desire underneath. (PLUS the fact that not all people are lucky enough to even have a sibling even with a very strong desire for a real-life incestuous relationship with one in the first place.)