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 heard it’s just a placeholder for vanilla porn.
It’s literally just an easy way to search ages. Stepsiblings means you want 2 actors who are younger. Mom/dad specifies older. Etc. It’s more to do with how searches catalogs and mixes terms like “older man, younger woman,” which pulls up often the same results as “younger man, older woman.”