Beyond the Grave by Jude Watson
Amy and Dan are hot on the trail of the fourth Clue, but their ruthless relatives will stop at nothing to eliminate them from the competition. Their hunt leads them to an ancient city that has been a hot bed of Cahill activity for centuries. All that stands between the siblings and the Clue is a cryptic riddle, a missing relic, and the secret of their grandmother's past. The clock is ticking for Amy and Dan, but can they really solve a mystery that has eluded their ancestors for generations?