Books in the Yshan Kings Trilogy series

The rightful heir to the throne of the Galactic Empire has been kidnapped and with it, the Star, the sentient crystal that has guided the Yshan Dynasty for several thousand years. Kyle and Sophie, the twin children of the Emperor have come to Earth to seek out the Prince and the Star and they are joined by Josh, Charlie and Henry, but they must overcome treason and great dangers before teh rightful heir to the throne is found and rescued.