Ooh, Spun, that's a great idea. I haven't yet seen the Japanese movie, but in the American version, Jennifer Lopez and Richard Gere managed to be steamy but not go over the line in terms of chemistry. (At least, so I inferred, since Richard Gere had Susan Sarandon back at home, cast as his wife, in the movie.)
I remember that the Duchesnays did a West Side Story free dance in which they had to say they were skating as Anita and her brother Bernardo...well, it worked, and why not? They also did some non-romantic dances, notably Missing, which was a political statement that could have depicted any man and woman. I don't know whether the musical limitations nowadays (music choice has to be closer to dance-related these days, doesn't it?) might make it harder to find stuff for a brother and sister to do, but I'm sure someone will get ingenious.