Time to update the tag to incest… seriously though, why is that genre popular in Japan??
...What?
It's a hugely popular genre in porn everywhere, aren't there even studies about it?
I think the most popular theory is that a lot of porn is really damn fast, they quickly go straight to the fucking and thus there isn't a lot of time to establish relationships, and i guess being married or dating is too vanilla, so incest gives some edge to it and people just accept in their minds that through familiar bonds these people already know each other for years and have some deeper bond beyond just screwing each other on sight.
It's basically a replacement for actually introducing a relationship and yet getting things done fast...some also just like incest.
Beyond that, step sister or step brother is insanely popular, again, articles have been done about it, it's just the author either liking it too or going with a trend.