Even if a person develops diabetes, many times he or she can control this disease with diet. You can not base everything on genetics. There may be additional factors why your relative is a diabetic. Perhaps that person is overweight or sedentary, meaning that they rarely exercise. Many Diabetics are able to control the disease with diet and exercise.
Some patients are even able to avoid the use of insulin injections or other insulin medications.
There are many diabetics who were even taken off of insulin after they modified their eating habits and became more active. The key to dealing with this disease is early detection. You can find books on diabetes at your local library. Try looking up diabetes in an Internet search engine. You can ask your doctor.