My initial instinct to this headline was: Why would you ban that? After reading the article, I might agree this is a reasonable option.
From my perspective this type of porn was basically just porn with a bit of story attached to it, which imo elevates it slightly. Though I was mainly thinking about the step-sibling variety. From a male perspective the stepfather variety always gave me the ick, with the stepson dynamic lying somewhere in between. But I was looking at it as someone without any step-relationships in real life.
Now, after also looking up the prevalence of step-relationships lying between 10 and 40% depending on what you count, I am a lot more vary of this trend in pornography from a societal perspective. Even besides the more insidious possible effects mentioned in the article, I can see how this type of fantasy can give the (presumably majority male) porn audience wrong ideas. Like considering every woman a possible partner instead of just peer, even in the wider context of family.
In theory I agree with these kind initiatives to try and at least steer things in the right way but I also realise how often they fail. And who knows the trend following such a ban could be worse.
Ultimately, you are only treating symptoms here and ignoring the societal problems that lead to this being a thing in the first place (loneliness, fewer human connections, rise of individualism, etc.)