When they stop taking responsibility for their own actions (if they ever do) and blame one another for their mistakes. Owning up quickly is less painful than dragging out a petty finger pointing session.
I think this probably true of non-families as well.
The great thing about fandom/internet friends vs. friends you meet out IRL, is that when you get to know people for the first time face-to-face, there’s this awkward process of trying to figure out juuuust how much of a dork they are, and how much you can nerd out before you scare them off. Like, you don’t wanna break out the real freaky shit right off. There’s always the impulse to hang back a little, as you try to gauge just how into a thing they are. But with fandom friends? You fucking met them in the garbage heap. You knew their fucked up narrative kinks before you even know their real name. They are screaming their passions into the void. Your friendship comes pre-loaded with already knowing the exact depths of each other’s depravity, and any ordinary-people-shit you have in common is just a bonus.
The Doctor Is Out 