Quote (iCK @ 29 Apr 2014 15:24)
This guy is a liar D:
Stupid.
Sike
It's true
Or maybe
It isn't

Obviously it isn't true. The Novel/Manga sales would plummet if they got rid of one of the siblings. At least before the very end of the story. Tatsuya and Miyuki make the story, the rest just give it more plot. I expect one or both of them will die in the very end of the story. But until the last chapter of the story, they'll both live.
Tatsuya is virtually immortal because of his reviving thing(at least what i get from it) and Miyuki is an absolute prodigy with magic. No one in the show right now(except maybe master Kokonoe) can even compete with either of them. Saegusa, Mari and Katsuto's powers have barely been touched on, so they may put up a good fight against them. But i do not believe any of them could defeat them right now.
*this is all based on the Anime and Manga. The manga is only a few episodes ahead of the anime, so not much more information is there. Planning on reading the Light Novels before long though.