Until I saw your post explaining some potential "whys" for your hubby's actions, my first response was:
"After I spoke with a good divorce attorney, I would go get my dog back." (The murder option someone mentioned, while effective, wouldn't do you or Louie any good.
)
If your hubby action's were in fact aligned with the motivations you mentioned, it won't be hard for him to see the errors of his ways. And while it's sad that your nephew just lost a dog, it should make it very easy for him to understand why you would be so saddened to in effect "lose" yours!
They can go save some other deserving soul. Louie, however, is YOURS! You can look at this as a "brief vacation" for Louie.
If he can jump in their van to leave, he can jump right back in it to come HOME!!!!!!!