I don't know if it's true in general. I don't have the data on that. But I do remember some videos in which "But I'm just your step-sister" was said quite sarcastically.
When I was 12 I got a step sister that was 17-18 and I definitely had a thing for her.
I also have no good data on how many people have step siblings. But it has to be quite a lot if it's a market for one of the biggest porn genres.
if they dropped the step I would be 100% out.
It really does make it a lot weirder. That's why they don't do it. I probably still would be in though. I can ignore the title and dialoge if needed.