@Marmot For some reason incest is frowned upon even in fiction. Often more than rape and other far more understandable things. I don't think fiction should be restrained at all, but there are legit people having to put disclaimers on content they make to avoid some idiot getting upset and ruining their business.
This is why there are a whole bunch of things on patreon and such that say "step" or "roommate" instead of what they obviously are. It isn't even limited to porn like one would expect. There are just people out there who don't even like the idea of it existing to the point where they interfere in ways that they have no business with. Paypal and such not allowing payment for instance.
Honestly its sickening that people can't just do what they want when its literally hurting no one. The general war of porn, video games, and entertainment in general is just appalling.
Anyway this is just another case of "you can't do that" usually explained away with bullshit like broadening the audience or age ratings. That or the writer just wanted to sneak in a blonde loli and had no better way to do it.