I wasn't aware that if someone puts their hand in the air in front of me, it's a huge insult to not suddenly grab it. Isn't that the equivalent of saying it's rude to decline an invitation to a party, or to not accept a gift? An outstretched hand is an invitation, nothing more. No-one has the right to decide how and when we should react by making a specific gesture.
It's not rude to choose not to do something if it makes you uncomfortable. For example, although it's normally rude not to make eye contact with someone with whom you are speaking, I have taught plenty of students with various reasons for not wanting to do so (aspergers, a lot of the time). Nothing wrong with that, if they are genuinely uncomfortable by it. It's the same as if friends go to kiss you--I only feel comfortable being that intimate with my husband. Yes, I'm a prude, but no-one has the right to force me to kiss them. That's what we call assault, after all. People of foreign cultures aren't expected to change their entire lifestyles if they move countries if they aren't used to it and are more comfortable with their own way of living, even if it affects others a little (e.g. religious dress). For goodness' sake, they used to force homosexual people to marry or undergo 'therapy' to 'fix' them due to what was culturally considered acceptable at the time. So just because something is culturally encouraged does not mean it should ever be mandatory. Good etiquette is to strive to make others feel comfortable within reason. Not wanting to shake hands is hardly the end of the world, and very much appropriate for someone who is uncomfortable and to whom it has been clear that they are considered not a friend but rather a sexual deviant of some unsavory sort.
That said, I am not American (in case my spelling didn't give that away already), and I'm aware that hand-shaking is a stronger cultural obligation over there, especially down south. Even so, how can it be considered rude to quietly not be forced into something you're uncomfortable with? I apologise if there is a cultural reason that I am misunderstanding here, but I strongly feel that no-one should have the right to force another's actions except for safety or where outlined by law.
Yes, your brother had a lapse of judgement going over to your uncle's house. But one mistake does not always mean that you have to endure unpleasant situations beyond what you can bear. People sometimes go to events and realise for various reasons they don't wish to stay, and can perfectly politely quietly thank the host and leave. And having guests is a two-way street, and part of that is the host's obligation to making sure the guests are comfortable. Your uncle seems to have purposefully done the opposite of this, although again there is no first-hand account to confirm this. Does your uncle normally shake hands with people as they leave? Was it everyone or just a selected few, i.e. did he put your brother on the spot with this stunt?
Again, I apologise because my view is contrary to so many others', and no personal offence is meant. I only wish to put forward my ideas here, nothing more.