There are three kinds of diabetes:
(1) Juvenile onset diabetes
(2) Adult onset diabetes
(3) Diabetes insipidus.
The diabetes that you refer to is genetically caused, thus, the lack of insulin production to convert blood sugar into useful energy packets. So, patients inject insulin to help their body out.
The diabetes that the brother's talking about is (2) where improper diet and too much intake of refined carbohydrates exhaust the mechanism by which insulin uses to transport free flowing sugar in the blood into our cells. So, people with (2) take tablets that help their insulin to do the job.
The brother is right.
I wouldn't say irreponsible or neglectful, but maybe not careful or uninformed about the effects of too much sugar and refined carbohydrates do cause adult onset diabetes (2) in many teenagers.
If you'd like to know more, please go to any low carbohydrates diet advocates website. I'm beginning to sound like a dietician here...