I mean to be fair, its ALWAYS been this creepy. Go back and watch some of the "classic" porn from the 70s and 80s. Its always dudes trying to get with a stepsister or stepmom or aunt.
Porn writers want to create scenarios they think seem realistic to the consumer. And what do consumers have? Step families. No one really pays for a pizza with a blowjob. No lifeguard saves the blonde hottie and gets sex in the locker room as thanks. Gym teachers don't seduce the cheerleaders. And shit like Girls Gone Wild and HBO AfterDark are just no profitable when Reddit has a hundred+ subreddits dedicated to every dark disgusting thing people think of when its 1am on a Wednesday and they can't get to sleep.
I think it has to do with the taboo of it all. It's wrong, and therefore that add's to the naughtiness of it all. I mean there are casual uses of incest in general real world relationships. Ever have a girl call you Daddy in bed? Or called your partner Mommy? It's just a hot kink I guess.
It exists in every single aspect of porn, from the normal videos on the Hub, to the stories on Literotica (where incest is the most popular catagory by miles), to Hentai and even Japanese adult videos (JAV's).
Most Western porn these days at least calls the people "step" siblings or kids to add a slight degree of separation from it being true incest. And also probably to protect themselves in a legal loophole.