Don't Say A Word - Debby Meltzer Quick

The Reed children Kaya and Graham, and their mother are devastated when their father suddenly decides to leave the family, and go out into the world to find himself. But as time goes by, they must move on. What could be better than starting high school with your best friends, making the cheerleading team, and having everything you ever wanted? It's the best time of Kaya's life. But then something changes. At first, Kaya doesn't think the messages she's hearing are a big deal, and she's baffled by her brother being so concerned. But soon it becomes obvious that Kaya is hearing things that no one else can hear. This doesn't cause her too much distress, until one day she hears something so terrible, she can't ignore it, and neither can anyone else, including the authorities. Is Kaya really as ill as everyone thinks she is, or is something else going on? Kaya's brother, Graham, makes it his life goal to find out, before it's too late for his little sister.