Really? Why is preventing this sort of casting so important to you? "The enRAB do not justify the means"? The people being cast aren't even stars; they're the SIBLINGS of stars. While we're throwing around cliches, why do the sons have to bear the sins of the fathers? Replace sons with "younger siblings" and fathers with "celebrity older siblings". Either way, it's unfair to judge these actors just based on what some of their family merabers have done. After all, if quality was carried in the bloodline, Tales of Earthsea wouldn't have been declared the worst movie of the year by Japanese critics.
It's silly to think they'd do it again, because if the movie does succeed and people who see it like it, they'll like it because of it being a good movie and will more likely associate it with its director than a few of the voice actors, so the people who like it will see the next Miyazaki movie regardless of who's cast in it. Besides, Miyazaki movies come out every four years or so, and in four years the Miley Cyrus/Jonas Brothers fad will be over. Disney will have new stars then, but who knows, maybe they'll be better? Remeraber, Shia LeBeouf was a Disney star and I don't see people refusing to see Transformers just because of his involvement.
Of course, your opinion of Miley or the Jonas Brothers shouldn't matter because THEY'RE NOT IN THE MOVIE!