While the risk of type 1 diabetes is certainly genetically determined, the rate of this condition is rising, which would not be the case if genes were the only determinant.
Because it was assumed that sugar was not causal, it wasn’t looked at till recently, but this 2014 study found a nearly doubled rate of progression in individuals at highest genetic risk with highest sugar consumption.
http://www.ncbi.nlm.nih.gov/pubmed/26048237

A mere doubled risk is of course not evidence of causality at all, but is evidence of either sugar or some other lifestyle or environmental factor associated with high sugar consumption (e.g. nutrient deficiencies, food dyes, tooth decay bacteria) hastening the decline of beta cell function.