People who are into step sibling porn — did you grow up with a sibling (or close relative of a similar age) of the gender you're attracted to?
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.
You should do a poll, if you get enough comment responses to be statistically significant it would be a pain to sort through them. A poll would also make people more likely to respond, as it's less effort.