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.
As long as every other sentence isn’t “daddy” or “step bro” I’m fine. I tend to fast forward over the obligatory 10 minute blowjob, too.