Immediate family includes mother, father, and children only.
In spite of it's strong genetic component, most people with Type 1 diabetes are the only one's in their family with the disease. This helps explain the shock parents feel when their children are diagnosed, since there is usually little hint of risk. Scientists are learning more and more about the causes of Type 1 diabetes. Less insulin in the thymus may mean that cells that react to insulin are not weeded out as the should be. See The Risk of Developing Type 1 Diabetes for more information.