Diabetes in Infancy is a bit rare (as compared to other age groups). I hope your niece will be fine after rehydration therapy in hospital along with Insulin
At this early age the diabetes could be due to :-
1) Mutations in the receptors - Chances are less in your niece.
2) Pancreatic abnormalities - See if her stools are abnormal. Also better to get a sonography(ultrasound
3) Type 1 diabetes
- This is more likely in your niece's case as she was presented with Coma
So we come to Type 1 diabetes in Infancy which could be:
a) Temporary - After some days, she will become normal and might not need insulin for few months to few years. But in future, she will again need Insulin.
b) Permanent - The insulin which is started right now, will continue life long. This is more likely in your niece's case.
In short, she will be fine with regular administration of Insulin. Please ask her parents to learn the Insulin administration technique from their doctor/nurse before her discharge from the hospital.
Hope this helps. Please keep us updated, if it's possible.