Your relatives are basically the only person you shouldn't fuck

Isn't that the explanation you're looking for?

It's mostly just click-bait and a thin-veiled plot.

I guess it works, because it plays with the taboo, giving it a bit of the thrill of the forbidden fruit, which maybe runs deep in the psyche of many people?

I think its mostly annoying. Maybe troublesome for some, but that can be said for most porn if you're not mindful about your consumption.
But it can also just be a fantasy that works for porn but doesn't have any real-life consequences. In both regards it's just like MILF stuff and a lot of the other problematic power dynamics that fuel a lot of porn plots.

I'm totally opposed to governments trying to censor things that they mark as abhorrent and try to stifle public outrage. That reeks like a fascist tactic to me.

(Disclaimer, that I do draw a line for stuff that actually harms people (and animals) because of the circumstances of its production. Obvious exploitation and non-consensual activity (which automatically includes children and minors) has to be illegal. But a plot is just fantasy and shouldn't be censored. )

I think it's popular because it has hot girls. If you skip the first 5 mins there's likely no mention of them being family. It's also not like they're running a montage of them getting up together before they get to the fucking.

Cast hit girls for step porn. People watch hot girls getting railed. Wow people really like step porn better make more and hire the hottest girls...rinse and repeat.

The step thing does nothing for me but there is a production company that I think does have good filming that make step stuff I watch it but skip to the end of the bj. I generally like smaller boobs and hate fake tits the step niche has that.