Hello, I'm Dr. Deb.
I recently came online and see that your question hasn't been answered. I'm so sorry that you've had to wait for a response, but if you still need assistance, I'd like to help if I can.
There may be a touch of separation anxiety if he's worse while staying with you but most dogs with this problem only behave this way while people are not in the house.
Sometimes it's difficult to distinguish between a behavioral problem or a medical one.
For example, I've seen some dogs with hypertension behave in some odd ways but this condition is usually secondary to either diabetes, Cushing's disease or kidney issues. If this were my case, I'd want to rule out the first two conditions since some of his behavior (increased thirst and appetite) could be seen with either one of them. If he hasn't had blood work done recently, then this may be something to suggest to your sister. Diabetes is relatively easy to diagnose. While Cushing's Disease may be suspected on routine blood work, often additional testing is required.
If he's diagnosed with either problem, then I'd want to measure his blood pressure and start him on medication if elevated.
If blood work is normal, then you may be dealing with a primary behavioral issue. Some dogs are just born very anxious and high strung which makes living with them often quite challenging. I liken them to OCD or ADD conditions in humans alhtugh this is often difficult to prove.
Changing or modifying this type of behavior is not at all easy but I do have a few suggestions which may help while he's with you. Of course, his behavior may be so intense that these suggestions won't be effective for him but they won't be harmful and might be worth a try:
1. Purchase of a product called DAP which comes in a diffuser or collar. These are often available at pet stores. They contain natural pheromones which are designed to instill a sense of calm and deal with anxiety. LINK
There's also an oral product called Composure Chews which is supposed to help in the same way.
2. Thundershirt which is also available at many pet stores.LINK. I've had a fair number of clients use this product and I've been amazed at the results. Of course, not all dogs will respond as we would like, but this is definitely something to consider. It's similar to the concept of some autistic children who respond to swaddling..
3. Over the counter Benadryl at a dose of 1 mg/lb twice a day might make him a little sedated and take the edge off. This is an entirely safe drug which could be given every day, if necessary.
4. Melatonin (the same supplement that you or I would take) at a dose of 3-6 mg twice a day. Some dogs will respond to one dose but, unfortunately, others will need to take it for longer periods of time before you see calming effects. So this may not help you now, but it would be worth considering.
Quality control is often a problem with supplements; Nature's Bounty is a reliable brand name.
5. I don't know if your sister would consider drug therapy but it can be very effective for problems such as you describe. I often use Xanax for this problem; it gets metabolized pretty quickly and the effects can often be seen within 30 minutes. You could use this drug on an as-needed basis or every day.
Prozac is another possible drug to consider but it takes time to build up in the system and needs to be given every day....you shouldn't stop and start it, in other words.
I hope this helps although, again, my apologies for the delayed reply. Deb