they used to cut the Nicholas Brothers out of their films in the southern USA, where audiences would be outraged if whites and blacks appeared in the same film. So they'd never have anything to do with the plot in the rest of the film, they'd just turn up and do their big numbers and vanish again. But they were fabulous - the best of them all, I think.
Funnily enough Chris I saw that clip a couple of weeks ago, Loved it, its different,
My father in law was a great tap-dancer & his favs were "The Clark Brothers" who were very much like the pair in your clip jno. but a lot younger and I think there was one who was about 12. Its nice to see more clips on AB.