I'm thinking of not returning my adopted dog when the family asked for it back, is this going to be a problem if I tell you?
Why do you want to keep the dog? Is there something  about this that is not truthful?
The dog was expensive and I've taken care of it the best way I can
I understand wanting to keep an animal that you spent time and energy on, but you should consider their feelings. They probably miss their dog terribly, and it would be wrong to keep it from them. If you haven't had the dog for very long, you should let them have their pet back.
I will return the dog to them.
This is a sensitive topic and the dog is theirs so yes please do return it. They will be distraught if you don't. They need closure as to where their dog is at all times to be happy.