Changing MCs in the middle of the story?

[warmageaubrey.livejournal.com]

Okay, I'm already considering my NaNo plot, but... well, the one I've come up with before requires changing MCs in the middle of the plot.
You see, it starts with one character, Fiachra, who's sort of a rebel-leader person. All sorts of bad stuff happens to her, and in the end she has two sons who are half-brothers. Well, the brothers don't even know that the other one exists until they're somewhere between 7-10, but at some point they're gonna start hating each other. And then, 'bout halfway through the book, the MC is gonna switch from Fiachra to the brothers. Fiachra is still gonna be there, but unlike in the first half, the brothers are gonna be the main focal point.
Now, my question is... would switching MCs in the middle of the book be way too annoying? I keep thinking that people will want to stay focused on Fiachra, and the brothers will just serve as an irritating interruption... but I really think that at the point the switch occurs, the brothers will be much more interesting to follow than their mother. And I realize now that I'm babbling, so I'll shut up.