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.
I just like that it’s taboo, but not outright abuse. I’d love to find another subgenre that had a decent amount of available content. Suggestions?