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.
There are some creators on PornHub and the like that seem to be doing SEO with their titles. One of their videos will refer to the woman as “step sister” the next video will use “roommate” another will use “cheating wife”. None of the videos have any dialogue so I think the channel is just trying to show up in as many search results as possible.