The London Times published an article about the higher incidence of diabetes during winter. It cites a pretty large research study of 31,000 children in 51 countries from the Diabetes Medicine journal.
My daughter and wife were both diagnosed in summer. Which doesn't jive with the study.
I'm not surprised about the winter correlation. Type-1 diabetes has been found to be (sometimes) triggered by external stress. Certain viral infections have been identified that set off the autoimmune response that can result in T1 diabetes. Winter, after all, is a perfect breeding ground for infection when everyone huddles around indoors in sniffle-ridden classrooms.