I have a pet hypothesis that, on average, those into step porn are more likely not have grown up with a sibling of the gender they fancy. I reckon those of us who did are more likely to repelled by anything alluding to incest.
People who grew up with ACTUAL hot step siblings notwithstanding.
We’ll see if this is going to be controversial or not but isn’t the entire “step” -part of it there only to give people plausible deniability about incest accusations? It’s incest these people are turned on by. To my knowledge having sex with your step-anything isn’t incest because you two aren’t related.
Either way, I think that most people who like watching mom-son, brother-sister, dad-daughter etc. porn usually aren’t sexually drawn towards their own family. They just like the idea of sex between family members in general but not their own.
It’s not REALLY incest that turns them on usually. It’s the deep taboo of the potential act itself. Thus plenty of people are turned on by the thought when it involves others, and plenty more are turned off when it comes to their own family. Similar can be said for a lot of sexually charged taboo subjects.