They Never Came Back by Caroline B Cooney

Cathy never expected Tommy to see his cousin Murielle in her face. But he does. In the middle of a crowded lunchroom during summer school.

His outburst causes an avalanche. His parents--mom especially--are convinced Cathy is Murielle. Five years ago, Murielle's parents fled the country before they were arrested for stealing money from their clients, and Murielle was left behind. Murielle was then placed in foster care, and that was the last Tommy and his parents saw of her.

Cathy is adamant that she is not Murielle, but Tommy, his parents, and their classmates aren't so sure. Then the FBI gets involved and suddenly all secrets must be told.

The book is told from Cathy and Murielle's points of view. Cathy's takes place in present tense, and Murielle's takes place in the past. The story is deep and heartfelt.