My Brother-in-Law does not Believe that Capsuleers Exist.
He says that the “Sky People” are a Myth told to scare Naughty Children.
I am Unable to provide any Evidence that is sufficiently Convincing.
I am at a Loss as to how to Proceed with this Situation.
Does Anyone have any Suggestions 
Put his butt into a Venture and drop him at an asteroid belt in Uedama. He’ll soon learn about Capsuleers 
Have you considered that he might me a stupid person?
Why does it matter to you what he believes? What concrete problem does his eccentric beliefs cause?
When Someone utters something that is Demonstratably False within my Presence, it tends to create a Sensation that I Experience as a Buzzing Noise in my Brain, which Annoys Me, and I feel extremely Compelled to Correct Them.
Plus if he Brings it up when I am Visiting, then my Sister will also start Arguing, and then the Two of them leave the Room to argue in Private, and I am left to entertain the Children and I find it A Bit Exhausting to play the role of “The Fun Auntie” to multiple children Simultaneously.
You think they leave the room to ‘‘argue’’ in private.
That’s a curious belief on its own.
“Sky People” seems to imply he doesn’t think stuff exists outside Kaztropol?
Have you tried putting him on a shuttle? I mean, even if he just disbelieves in capsuleers in particular it shouldn’t be hard to set up a few meetings with some, or even demonstrations of the tech.
If he’s still a disbeliever after seeing a few things with mk. I eyeballs, maybe make the next big project a sound-cancelling device so you can put him on “mute” when needed at social events and nobody has to listen to him?
I’ve met him. He’s possibly still a touch under the influence of culture shock, being from one of the pre-industrial population groups. Lots of those exist on many Covenant planets, a situation that Omir’s lot find convenient for their purposes. Kaztropolis happened to be located near to one of the groups, there was some… unpleasantness, and now they’re a productive sector of Kaztropolitan society.
Synthia’s sister Connie, have you met her ? She’s less talkative than Synthia, and far more interested in building things. She’s been busy improving the circumstances of the people, building such things as roads, aqueducts, temples, houses with indoor plumbing, all that sort of thing. Anyway, Connie is this guy’s wife, and it was somewhat of a politically arranged marriage, at least at the beginning. Though they do seem to actually care for each other.
So, it’s also possible that like most men living in vaguely matriarchal societies, he wants to at least occasionally be appreciated for his mind, and not just his admittedly impressive physique. He is well over 2m tall, and as such towers over Connie (and Synthia) like a colossus, as she is only 1.58m tall.