They both have type 2 diabetes and they are the only ones with it.
It sounds like you have a significant family history of diabetes. You did not specify whether the diabetes in your family is type 2 or type 1. Type 2 diabetes is a complex condition caused by genetic and environmental factors. Like all common diseases, there are many risk factors for diabetes. Rarely is one factor alone responsible for causing the disease. Your total risk is made up several different risk factors which include family history, genetic risks, weight, physical activity, history of gestational diabetes, and diet. Diabetes can be diagnosed at different stages of life depending on the combination of risk factors.
The more risk factors that you have for diabetes, the greater your risks. DNA testing for one confirmed genetic risk marker, deCODE T2, for type 2 diabetes is available through DNA Direct. People who test positive for this marker are twice as likely to develop diabetes than those who do not have the risk marker.