Not to nitpick but sugar doesn't really cause diabetes, it's carbs and that's only type 2. Also as diabetes is researched more and more it's being shown that genetics also play a large part in type 2 diabetes.
It is passed through genes, but it's the same with liver disease and the like, some people are more susceptible due to genetic factors so they should be more careful.
Everything you eat is broke down into glucose and stored in the liver before being released again for energy. When eating sugar, you are basically just having raw glucose which means you are unaware how much more glucose needs to be stored by the liver. This causes strain on the system.
Also, carbs are a very good source for glucose