My mom's identical twin sister has had diabetes since her late teens (now 50), but my mom doesn't. Why?

Jerie was diagnosed around 18-19, but Terie has never had a single symptom. They are indeed IDENTICAL twins, so why don't they both have diabetes?

Identical twins share the same genes, but not the same exact environment after birth- even though they grew up in the same house, went to the same schools, ate the same dinners- etc. They did not always have the same diseases, nor come in contact with the same viruses. There are many theories about why juvenile onset diabetes occurs, but one of them is recognizing the influence of a viral infection. They may have both even been sick at the same time, but in the case of your mom's twin sister- the infection affected the Isle of Langerhans in her pancreas, and your mother's infection did not. The injury and death of the cells which produce insulin resulted in the one twin becoming diabetic, and the other not. I understand the confusion, because we think that since they are identical twins, that means everything should be the same. But in spite of the shared genetics, they are still not the same person- and it's the differences they experience as two different people that can make the difference in their health.

Just because your mom and her sister are identical, doesn't mean that they are alike in every day. The only thing that they share are the same chromosomes. Identical twins don't even had the same finger prints. Each child developed apart from each other in the womb and will have different health problems.

You aunt is most likely type 1 ans type one is only 3-5 % chance heredity

it is a problem with the pancreas - maybe from environmental things that the one came in contact with, but the other didn't. hope this helps.