THE 100 YEAR OLD SECRET by Tracy Barrett
Xena and Xander Holmes have always been very observant. They can notice things about a person and deduce his occupation or details about his personal history. It is their favorite game. But when they learn that they are the great-great-great-grandchildren of the famous Sherlock Holmes and that they have files upon files of his unsolved cases at their disposal, they decide to upgrade their game to solving real-life mysteries. Their first mission: to find a priceless painting that disappeared over a century ago.
If you like the Thirty-Nine Clues series, you need to check out the Sherlock Files. There are plenty of clues to unravel and a decent amount of suspense in this mystery series. If you like character-driven mysteries, however, you will not find a lot of depth here. For character-driven Sherlock Holmes-related mysteries, I would instead recommend the Enola Holmes books by Nancy Springer.