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 a step-whatever motive in the porn videos. Like, step-sister gets stuck in the washing machine for some reason and is subsequently banged in the rear by step-bro while she’s unable to free herself.