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.
No, but I am turned on by the idea of a smoking hot woman I am living with showing up randomly in my bed demanding consequence-free sex.