I'm pretty sure he's a flat-earther.

Hygienist talking to me... so, tell me about yourself blah blah blah

me: Well I worked for NASA as a Chief Scientist before founding my own startup... sold it 5 years later... blah blah ( erg, swallow, bleek )

Hygienist: So... you worked at NASA... tell me the truth... we faked to moon landings, right?

me: ??? ( OMG. she is serious! )

He's visiting for a week to help with a family thing and we're all on our best behavior. (My best behavior isn't all that good but it's my best.)

At dinner the other night he mentioned that he believes that demons take over people's bodies and it happens a lot. I was proud of my response. Longish pause. I asked, "Like in the Exorcist?" He said yes. I had a few more bites of food. "Like you think the Omen could really happen?" (Very neutral tone, not excited, staying calm while thinking inside holy shit this is next level nuttiness.) He responded, "There is so much evidence! Like the UFO stuff that's just been released."

Someone needs to mirror reality to these people. They are in a bubble.
It might not work but we have to try. Gently, maybe. Somewhere in there is a human being trying to climb out.

or "You're really not that stupid, are you?" would have popped out of my mouth before I could have found a politer way to say it.

...when I had to bite my tongue when parent said something really stupid.