I check and watch for some kind of substance abuse or other problems in your nieces life. There might be a reason she is off her game.
Or may be the coach is the problem. But I have seen many parents write of their children's troubles by trying to blame some one else.
But if she's “off her game” why is she still starting and playing at least 80% of the game? She played more in this game after the e-mail then in the last game.