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’ve always suspected that there’s some coincidence whereby a lot of people have suddenly started watching some step-sibling porn simply because it’s among the first videos they see and not because they’re into step-siblings, then whoever decides what genre of porn gets made has noticed and assumed it’s suddenly more popular for a reason, so it gets made more often, then there’s a feedback effect.
Edit: that said, I don’t know what kind of porn anyone I know watches, if any, but I’m sort of vaguley aware of this step-sibling meme.