You will find yourself wanting to make regular visits to your 'now' home for family visits.
You can pick up her insulin while there. Your mother-in-law can bring her medical supplies to her if she is willing. Bring her glucometer and strips (or whatever she uses) along and bring a few months supply.
Hospital, yes...mainland. She can get there by plane in daylight hours, if needed.
Nurses tend to be extremely careful.
Diabetics generally know their own limits.
You as a husband should know her needs in case of insulin reaction. If you both know it, she will be fine. Do you have trouble in her daily life now? Diabetes is so common. My mother has had it, insulin dependent, almost all of her life.
Be watchful. If she is old enough to be out of the nest, she is old enough to take care as a diabetic
Again, go and enjoy
Have you been to Belize at all? How long are you staying?