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 make some good points, there could be a lot of factors… hell it could be that it took a while for the divorce and remarry rate to kick up and before that step siblings simply weren’t as common. There should probably be a good lag too between those numbers and when it started to become a cultural thing.
I would be interested in some deep research at this point.