Hate to say it but it may not be true. I have been diabetic for 20 years and I am on two insulins. Long lasting and fast acting. The reason for this I chose to keep on living the way I wanted to instead of how I should. Being a diabetic can do all kinds of damage to all of your body including your organs. Sometimes diabetics end up with nerve damaage, and it can happen anywhere in your body including your stomach. Please don't jump to conclusions. The first thing you should do is eat right and exercise. Whatever you do don't stop living. It does not have to be the end of your life. Once you have your diabetise under control, you will feel healthier, mentally and physically.
Whatever they are looking for is not diabetes related. Why don't you (or your brother) ask the doctors what the tests are for?