There are two types of diabetes. Type 1 is genetic. Type 2 is not genetic but develops because of environmental factors. This article is about type 2.
That's not strictly true.
Type 1 diabetes is thought to be a polygenic (many combinations of genes) disease, so more than one gene plays a role in it's development, and the genes can either be dominant or recessive. But, even if the 'right' genes to develop type 1 exist, someone may never become diabetic: in studies of pairs of identical twins, one twin having type 1 diabetes only puts the other twin at about a 30% risk of developing type 1, despite the fact that they both carry the same genes: so there's something else, probably environmental, going on.
Type 2 diabetes DOES have specific genetic mutations associated with many of it's cases. That said, you're correct that although having a relative with diabetes does put you at a much greater risk, because of the obesity related rise of type 2, we can mostly say that the biggest factor in developing type 2 diabetes is poor lifestyle in people with the genetic inclination.
I just want to clarify that it's not always 'either or' so to speak: there are type 2 diabetics who have never been overweight, and there are type 1 diabetics who are obese.
The stigma associated with being obese has gotten many people misdiagnosed with type 2 diabetes, and the association between obesity and type 2 has left some average-weight people undiagnosed for years, at the cost of their health.
Even if you're not overweight, asking your doctor to test your BG levels at your yearly visit, and if you have any symptoms, see your doctor sooner. If you ARE overweight and diagnosed with type 2, you should still ask your doctor for a c-peptide test at diagnosis to confirm that you are a type 2.