Paula Danziger: The Pistachio Prescription [2001] paperback

Cassie Stephens is thirteen, mousey and asthmatic. She's fed up with having to look after her little brother, being teased by her older sister, and ignored by her parents, who don't seem to stop rowing. The only thing that helps her to survive is her addiction to pistachio nuts - somehow they always make her feel better. But as the pressures of home and school mount, even the pistachio nuts don't seem to help...