You're being downvoted, but this meta-level problem really is
the problem to solve. How do you identify experts? How do you choose when to defer to experts and when to play it by ear? More generally, how do you decide how to make decisions?
I've thought about this a lot, but there's really no way to get around the fact that you'll draw insane conclusions if you start from insane priors. And you're kinda forced into having insane priors if your parents believe insane things. Either we force people to make the right decisions (as decided by [insert favourite entity here]), or we live with some people doing batshit insane things. Those are the extremes of a spectrum that includes many only slightly forceful interventions, but in a nutshell, that's the choice we have.