Incest is irrelevant in the world of fanfic. Pretty much any moral compass you have gets smashed after a few months of reading it. It’s like the wild west. There are no laws at all. You can write literally anything. I once read a fic where Sam was a sentient cinnamon roll being eaten and fucked by Dean in a café full of people. And it was told from the point of view of the sentient pastry.�
