My Sister Sam — Jean Ure

The first in Jean Ure's Foster Family series. Abi's parents decide to foster Sam, and Abi is thrilled. Sam is brilliant, bright and bubbly and Abi just knows they're going to be the best of friends. But when Sam starts giving everyone expensive gifts, Abi is suspicious. Where is the money coming from? Abi wants to think the best of Sam, but there are some things you just can't ignore.