Mate, NO ONE knows that Yui is immortal. Hell, not even Miharu knows she's immortal. Even if you do feel her "power"...the power is mostly from Chakra and Magic. The "not mostly" part has nothing to do with her being immortal. So, if you are referring to her, delete it. If your not, well, check the bottom paragraph.
If your not:
There are only a small band of immortals called "The Seven Immortals". Yui's brother is the leader of...guess what, seven immortals. :P
Uh can you just let us fight a bit? You always knock out the person who's about to do something interesting... or completely insane. Brothers fight. Its life. Trust me