Noob question about rapid weight gain in late 30's due to genetic predisposition.

I'm a total noob at this, and I don't really know the terminology, so anything is appreciated.

A woman I've known for a long time was relatively thin for years, but within one year at about age 36, she suddenly gained a significant amount of weight in just a few months. Apparently this also happened to her mother and one aunt. They all started Diabetes medication at around that time.

As best she can tell from talking to doctors and researching her own 23 and Me data... this seems to be some kind of genetic predispostion.

Is this plausible? Is it a reasonable occurrence for someone with diabetes in their family to suddenly have a big sudden weight gain in their late 30's?

And is there anything to look for in genetics?