The Sapphire Sword by Robert Fuller

Book 1 of The Sapphire Saga

Summary: 

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.
 
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 just might be successful.
 
If only they could find that legendary sword . . .          

Opening Lines: 

It was all the dog's fault--the altogether unexpected end of the world--but none of them would know this until after breakfast.