@Northman
I wonder if anybody would like to comment why this is prevalent?
We actually talked about this in my psychology class, when the whole incest-theme became popular with the first season of GoT (back in 2011).
And yes, I wrote "popular" because this is nothing new. I'm old enough to remember getting porn from Video-Stores, and if you checked the 'most rented' lists, there was ALWAYS something along the lines of "With my sister in the Barn - Part II" on those lists.
There are no real studies exploring this (at least I don't know about about any), but the consensus we reached back in class was twofold:
First, the most effective safeguard against incest in humans is the hypothesized Westemarck effect. Basically, it's the same reason marriages grow stagnant over time. The more time you spend with someone, the more intimately familiar you become, and the less interested you are to be intimate with them.
So, if you grow up in a loving and tight-knit family, you will most likely not have any interst in any of them. But people who were sent to boarding-schools, had emotionally distant parents/siblings or never reached intimate familiarity with them for any other reason, simply lack that "safeguard" and are more likely to fantasize about something like that. It also doesn't help that more and more parents simply leave sex-ed to the schools and desperately try to keep anything sexual from their kids.
The other big factor could simply be: It's a taboo.
Since all kinds of kinky porn become freely available through the internet, people tend to look for new stuff. Watching the 30th missionary-video in a row becomes simply boring. And this is still one of the least provided kinks... unless you count all the stepcest-videos. The people producing this stuff don't actually fantasize about their own family-members, but simply like the idea of SOMEONE doing it with family.