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.
“Well, I came here for big blonde tits, but you gave me step-sister porn…ah well. It’s pornhub. That’s 100% of the videos. When in Rome…”