"Fiction affects reality!" Of course it does. But 1) this is not a 1:1 impact/feedback loop; and 2) reality affects fiction first. The chicken comes before the egg. You can't have the latter without the former, and context is of utmost importance.

In order for a fictional falsehood to have a considerable impact on reality, 1) there needs to be a considerable lack of education about the falsehood, and/or 2) the falsehood needs to be dressed as a factual statement

This is why your Uncle Jerry keeps getting taken in by fake info about the 'rona: he knows very little about virology and the lies are dressed up in scientific language and official-looking graphics.

This is also why things like The Jaws Effect happened: most Americans didn't know much about sharks, and the film worked on a basic and realistic-seeming premise that sharks can be dangerous and scary. It was also based on real, well-known (but RARE) shark attacks.

This is also why there was a brief rise of sex toy related injuries after the release of 50 Shades. The general public is uneducated about safe BDSM (and safe sex in general) and the book worked on an unspoken premise that BDSM is inherently dangerous (it isn't).

HOWEVER, this is ALSO why, for example, Twilight: Breaking Dawn DIDN'T lead to tens of thousands of people thinking that it's okay for a 19-20 y/o guy to fall in love with a toddler. Our society IS fairly well educated about why this is fucked up.

It's also why, even tho FPS games are immensely popular all over the globe, few countries have the same level of gun violence as the USA. Most people know why it is wrong to kill, even if they play COD all day long. (US gun violence is, of course, a much more complex issue)

It's also why, despite the worldwide popularity of Game of Thrones, we aren't seeing a huge rise in IRL affairs between aunts and nephews or brothers and sisters. Most people already accept that incest is wrong & dangerous, or at the very least shameful.

Finally, for a fictional falsehood to have a considerable impact, it needs to be viral (like Facebook memes) or easily accessible to a massive population (like a blockbuster movie or TV show). And even then, as in the cases above, that isn't a guarantee of impact.

So in conclusion, no, people writing weird gross Gravity Falls fanfic for an audience of ~1000 readers isn't going to normalise pedophilia and/or incest IRL, and you need to calm the fuck down and click the x on your browser.