The 39 Clues: The Maze of Bones by Rick Riordan

The Cahill family has gathered in the Great Hall for the reading of the will of its matriarch, Grace Cahill. Grace gives her last instructions to her family via a previously taped video. She offers them each a million dollars or 39 clues which lead to a secret that will make the individual or team that finds it the "most powerful and influential human beings on the planet". Dan and Amy Cahill loved their grandmother Grace and miss her greatly. They know that she was secretly training them for this mission but they must pit their wits against other more savvy members of their family to find the clues.