The Sapphire Sword (The Sapphire Saga, #1)

To save the world from certain destruction, three siblings and their shapeshifting alien guide embark on an out-of-this-world adventure in search of an ancient, all-powerful weapon.    
 
Slugger, Flint, and Scout are always out exploring the mountains of New Mexico near their home. When a cherry-picking expedition puts them in the path of a peculiar pooch in peril, the three siblings have no idea what unstoppable calamity has just been set in motion.
 
The dog they call Robin Hood isn’t what he appears to be. And neither are the O’Ryan kids. In fact, they’re not even kids ! Well, not human ones, at least.  
 
Together, the three unlikely heroes and their trusty “canine” companion must journey the cosmos to snatch Earth back from the clutches of catastrophic evil. And they might just be successful.
 
If only they could find that legendary sword . . .