Missing Since Monday by Ann M. Martin

Maggie's taking care of her four-year-old sister for a week. On Monday morning, she puts Courtenay on the bus to school. That afternoon, Courtenay doesn't come home. She never made it to school that day. She's missing. The detectives looking for Courtenay have hardly any clues. Then Maggie starts getting threatening phone calls...and she thinks she's being followed. Whoever took Courtenay is after her, too. One thing's for sure: If the police don't find Courtenay soon, Maggie will be the next one missing. Book jacket.