Whisper by Phoebe Kitanidis
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Fifteen year old Joy Stefani has a strange superpower, inherited from her mother: she can tune in to other people's secret wishes. Lonely as it sometimes makes her feel, Joy's "Hearing" has always guided her in life, letting her know just how to keep her parents, friends, and teachers happy. In fact, granting people's wishes is what Joy lives for, unlike her mean sister Icka (Jessica), who uses the gift of Hearing as a weapon, to hurt people. When Icka's nastiness ruins Joy's birthday, she finally blows up at her sister... and wishes her out of her life. But when Icka disappears, Joy's mind suddenly starts picking up scary, awful wishes she never wanted to Hear.