You're right. It's not just legal reasons. There are some neat studies showing that if two people under the age of 5 are raised in the same household together, feelings of sexual attraction generally don't develop later when you're sexually maturing. So the porn somewhat follows reality in the scenario of step-siblings being more likely to develop sexual attraction.

Is this true? I’m surprised.

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.