I hope you took Jaybay's advice and sought vet help for your sister's dog.
What you are describing sounds like it could be spinal pain, or disk disease, or something simple like impacted anal glands. Disk disease can be a medical emergency, wheras anal glands could just be painful, (like hemmorhoids, or bad constipation) in humans. It is just safer for the dog to have him examined.
What you do is be a good dog-sitter and take the dog to a vet. Dogs don't "make like" they're in pain. He may just be unsettled from a change in living quarters, but he may also have something wrong. If your sister left you her vet's number, that would be a good place to start. Can you reach her while she's away to discuss this behavior with her?