The False Princess by Ellis O'Neil

When the princess of Thorvador turns 16, she is told that she is not really a princess. She is a decoy, a lowly peasant girl put in place to protect the real princess. Her name is Sinda Azaway, stolen from her parents to become a fake princess. When she turns 16, she loses her usefulness to the King and Queen, and she is cast out into the streets. Sinda soon realizes that the new princess isn't who she says she is. But can Sinda stop the plan before its too late?