Hi Tania,
I don't have personal experience with doggy diabetes but as a nurse I have plenty experience with diabetes with humans. I looked at a couple articles on canine diabetes and it is pretty much treated like it is in humans. There are different types of insulin: some are short acting and some are long acting. It says that canine diabetes is not treated with oral medication like they sometimes use in humans.
Initially, your sister's dog may need to have the inulin injections twice a day while they are getting the diabetes under control. Your sister should still continue the management with diet and exercise. As the diabetes becomes better managed, usually the insulin can be cut back and/or replaced with diet and exercise management.
Things like stress and infection can cause the glucose levels to rise in the dog's bloodstream. I am sure the dog's recent move has been stressful on the dog and as things settle down that may also help the situation.
Can your sister possibly hire a professional dog sitter or does she have a neighbor that can come and give the shot? Maybe the shelter will be willing to take the dog back for a month or to see how the dog responds to the insulin injections?
Has the dog already been started on insulin and how well is the dog responding to the treatment? Has your sister asked the vet his opinion about the dog's prognosis? Ultimately, she needs to do what's best for her and the dog.
I may have told you everything that you already know!
Good luck and I hope everything works out!
Claudia