39 Clues #4: Beyond the Grave by Jude Watson

Betrayed by their cousins, abandoned by their uncle, and with only the slimmest hint to guide them, fourteen-year old Amy Cahill and her younger brother, Dan, rush off to Egypt on the hunt for 39 Clues that lead to a source of unimaginable power. But when they arrive, Amy and Dan get something completely unexpected - a message from their dead grandmother, Grace. Did Grace set out to help the two orphans . . . ore are Amy and Dan heading for the most devastating betrayal of them all?