It is wrong to steal things. A living creature is not a thing. While the neighbors are technically the dog's owners, they are also her caregivers. It is their responsibility to secure a healthy and happy home, and assure the dog is being cared for.

If she is being mistreated they are not fulfilling that responsiblity- which they implicitly agreed to upon taking the dog into their home.

From my point of view:
You can't really "steal" a creature. You can kidnap it, you can rescue it, you can harm it, you can love it, you can mistreat it,you can care for it.

I understand with pets, all of these statements are not agreed upon- we all have different viewpoints.

But to respond to your question?

I think whether it is wrong to remove the dog from the home, and how it is done, would depend on what action or inaction you could live with doing or not doing. Ultimately, since it is a living creature, I would consider what is right by the dog AND myself and attempt to balance that out.

If I personally viewed crystal clear evidence of such repeated abusive treatment I would not actually consider the neighbors' rights as owners in the equation; in my opinion they forfeit such rights when they refuse to care for their charge.

Which isn't at all any particular recommendation, just the kind of factors I consider in such situations.