The Necropolis — P.J. Hoover

The situation in Lemuria is rapidly deteriorating. In fact teleportation between the hidden continent and the outside world has become so dangerous, all agents and their families have been recalled. Although Benjamin is pleased to be living in Lemuria full time, he knows he needs to find his last sibling soon. However, between classes, a murderous half-brother, and complications with his friend Heidi, Benjamin can barely focus. Besides, there's only one place left they haven't searched - the hidden continent of Atlantis.