Mission Atomic (the 39 Clues: Doublecross Book 4)

Thirteen-year-old Dan Cahill and his older sister, Amy, are running out of time. An exiled Cahill known as the Outcast has already recreated of history's worst disasters, and he's saved the worst for last. If Dan and Amy can't find and stop Outcast fast, he will initiate a full-scale nuclear meltdown.