I, too, have law enforcement friends who carry at all times. However, if not working, the gun is always covered --think wearing a jacket in warm weather, etc.
I do think there is an etiquette question here, and my answer would be that you are totally in your rights to explain to any prospective hosts that you will not be present if uncle is going to be carrying. It's bad enough that he was wearing it openly, but he completely stepped over the line when he started showing it around and discussing it, particularly to childrn. I don't even think it would have been out of line to have asked the host to require uncle to lock the gun in his care and to have advised that you would be leaving if that did not happen.
I also grew up with guns, but they were treated with respect and not as some toy.