As of January 2024, of the most popular 100 videos on Pornhub, 4.1 billion views were for step-incest videos and 3.3 billion views were for all other videos combined.
Not registering on the site, so I can't read the article, but the reason seems obvious enough. For most people, just looking to watch sex, it really doesn't matter if one of the actors starts a scene by saying, "Step brother, what are you doing?" For people who are into incest stuff, for whatever reason, they're going to be happy about it. It's a situation that doesn't cost extra money, all they need is a regular room in a regular house. So there's little downside for the porn studios, with likely bigger upsides.
I can tell you this: they know what people click - if it wasn't selling, they wouldn't do it.
Okay, that article is bullshit. It's popular because it appeals to the loses in our meritocracy and liberal democracy. How does he know this? He doesn't, there's zero data to back it up.