My Big Sister Is So Bossy She Says You Can't Read This Book — Mary Hershey

Something of great importance entrusted to fourth-grader Effi Maloney has disappeared and she's sure her big sister, Maxey, had something to do with it. Effi's no pushover, but trying to stand up to Maxey is like trying to stop an earthquake. It would be easier to deal with Maxey if Effi had a buddy. She hasn't had a best friend since Lola Jo moved, and she has so many secrets saved to tell a best friend -- including the most amazing idea in the world for a winning science project -- she's about to explode.

Effi's got to win that science prize, find a best friend, and get back at Maxey -- in just one week!