you could suggest that your sister buy the book "MINE!" by jean donaldson. it deals with resource guarding -the likely cause of the jrt's outbursts. it is really good about setting up a plan for counter conditioning and desensitization. i don't believe this book is more than about $10US so it might be worth a try!

ps: has he been vet checked lately? in cases like this a necessary first step is to make sure that the pup is healthy and there are no underlying causes for his snappy behavior.