Maybe it's only an American thing, but "uncle" can be an affectionate term for a parental male that isn't actually your uncle. He's just really close with the Thompsons and hence calls him Uncle Fred.
No idea on the second question. Looked like golfing clothes which would make sense.
---- I'd love to see you in the moonlight with your head thrown back and your body on fire.
I thought "Beavis" and "Butthead" were wearing tennis togs. That started the "out of place" feeling right at the start when the Thompsons appeared to be playing golf with them.