You asked how you deal with an uncle, who acts this way. You can tell him uncle x acts like this. We in our family are polite. We act differently. We do this. We do that. You don’t have to judge or berate his uncle, just make it clear the expectations in your nuclear family are different from how his uncle behaves.
My fil has many behaviours, which are not neurotypical. The family has always whispered about him being different but obviously being in his 80’s, he never had any kind of diagnosis. He is a kind and loving man. He drives me to distraction though. He works well with clear command. He sees nothing wrong with being told what to do.
He can also be a stubborn arse. Before I knew to just tell him, this is how it is, I have had two hour conversations with him. Such as to persuade him that he’s making a big mistake on redesigning his kitchen himself using the 1950’s style template, which was in existence and putting tall cupboards behind the back door, which he uses as his main entrance/exit, no work tops etc.
Fil also never learnt to say thank you. Dh didn’t learn either and I have also encouraged manners with him. Fil now says thank you to the person handing him the food. Not the person, who made him the food. He just can’t get it quite right, bless him (grrrr). It’s making me laugh inside to think it even though this royally pisses me off as I’m usually the cook. Dh usually hands out the plates. Fil also does not help out at all unless told, which I don’t tend to do as the stress and energy requirement of getting him to help and instruct him outweighs the benefit. So I think to he has baboon arms, which hang down very low to get a smile inside. Yes, he’s old. But until recently, he’s been in far far better health than me and I’d say even now he has more energy than me.
Dh was (and still) is massively protective of him until a few years ago when he finally HALLELUJAH admitted there is something different about him. Until then, it was always, that’s just how he is, his way, nothing to challenge, I’m making a problem out of it etc. The fights we had over him were fierce. Instead, we can now laugh about getting ourselves into a state about him together.
Dh now also accepts that much as I love my fil, since contracting ME/CFS, I find it incredibly difficult to be around him and do not to talk to him much. This may sound cruel, but it’s not intended to be. Fil is French and it’s a lot of effort for me to speak to him despite my being fluent. Fil isn’t very good at interpreting things and it’s very draining. He is a complete loner and has no conversation skills so talking to him involves a lot more than an idle chat.
What I’m trying to say is, please don’t oblige yourself to be the same with your bil as you are with people, who are definitely NT. I’m sure my fil doesn’t even notice my behaviour. Your bil is likely to accept what you are saying. So start making boundaries. One of mine was that on fils visits, he would stay 5 nights (as coming from France) and dh would be home. He has now decided the trip is too far for him so hasn’t been here this year. We have been to France though.