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.
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.