The Sapphire Song by Robert Fuller

Book 2 of The Sapphire Saga

Summary: 

Three space-dwelling siblings learn a shocking secret and embark on a treacherous, interplanetary mission to save an alien civilization from imminent peril.   
 
After wielding the unparalleled power of the Sapphire Sword to save Earth, Slugger, Flint, and Scout, spend their days cruising the cosmos and exploring the infinite possibilities aboard the Nautilus, their all-galaxy-class spaceship. Life is a sweet adventure—until their dad reveals a mind-blowing bit of intel that changes everything.    
 
Soon, they are whisked away to a strange planet in search of an ancient, all-seeing entity. Could the one they call Luminor shed some light on their father’s revelation?
 
Together with their curious new companion, the O’Ryan clan faces dangers that will test their faith and fortitude as a faint melody beckons them onward: Peace be still.   

Opening Lines: 

The lone robed figure glided over a floor of polished stone, its surface reflective as a mirror and smooth as glass, as if the figure hovered in midair above a ceiling far below, where stars winked and flickered like living things beyond an enormous translucent dome.