Blackfeather Chronicles Series by Ruth Lampi

Jon Gardner won an essay contest and the chance of a lifetime--a summer at the archeological dig at Alarna with the mysterious Doctor Blackfeather. Taking a train halfway across the world was only the beginning of the adventure. Tomb thieves and darker, older things are hunting the Alarnan ruins for artifacts as well. Jon and his brother Tam, the unusual Blackfeather children, and the dig artist Anna must investigate a theft and make unlikely alliances to protect what they have found.