Actually, it is partially genetic. Did you just not study genetics in school?
If the father has a dominant and a submissive gene of diabetes, and the mother only has submissive, the child will have a 50% chance of getting diabetes.
Then if that child got the Dd (one dominant, one submissive) gene set for diabetes, and he has a kid with another person with Dd, the child will have a 75% chance.
If that kid has DD and she has a kid with someone with dd (two submissives), the child with have a 100% chance of getting diabetes, but will be guaranteed to have the genes Dd. If he then has a kid with someone with dd genes, there is small chance the kid will have two submissives, dd, and will not have diabetes.
In short, Punnet squares.
I'm sure the family members you have that toss the word genes around at you would love to believe that.
But when every family has these cases, perhaps the coincidence that they are fat forgets don't occer to them. Its justg easier to blame genetics.
Even though all cancers are preventable and and all diabetes are 100% reversable.
But yes, self responsibility is harder.