I've been thinking about the steady increase of porno involving siblings, parents, or some sort of parental substitute (sexy teacher, lascivious nanny, etc). I'm convinced that this is the result of further atomization under capitalism. As social relations and active participation in community continue to degrade, essentially the only people you will have any sort of meaningful connection to will be your immediate family.
If you're lucky you picked up some friends at school and got along well with your co-workers, but it's very likely that you have no emotional ties or experienced any sort of physicality with anybody outside of who you grew up with. Of course you're going to end up horny for Daddy when everyone outside of the nuclear family is basically an alien that you're incapable of feeling any sort of connection to.
Somebody's probably thought about this more than I have, but I don't feel like I'm off-base in saying that incest increases in popularity as everyone's social circles continue to contract. Or maybe Freud was right and people have always wanted to fuck their moms, I don't know.
You're certainly onto something, but maybe you could address it from the other direction? Like somebody has to be funding production of quasi-incest porn. Maybe part of the answer is millenial failsons crushing on their wealthy boomer dad's 3rd wives.