It is just mindless pursuit of the forbidden for algorithm cheering reasons. And then you need to have had hot sisters to lust after growing up. With so many one children families you end up with a relatively small proportion of the population that could even care.
On the other hand, adding one incest adjacent keywords to the title, if it means 3% more clicks they'll do it
I mean, we all know all those step-siblings (or actual siblings) videos are pure bullshit, but you're right, it seems they attract some serious clicks. Or so I've been told.
Those click could be superstition about algorithm. I often click on them, it's not because the word "step" but the word is there anyway. Porn sites aren't sophisticated enough by and large to do proper a/b testing and find out if the word step make want difference. Especially when the actual content doesn't even play with the incest concept itself.
I get you, but you can't deny that "genre" still is a huge hook, also anime is full of that shit (the whole "they're not actually blood related" bs lmao).
I don't know, for me it feels chasing the forbidden for forbiddenness' sake. The biggest clue to that is how the actual content does really do anything with what the title promises. It doesn't change what's on screen