I'm not sure whether you can actually reverse this with crystals, or stop it from happening.
There's usually a deeper, karmic reason why someone gets diabetes, so I think it would be best to find crystals that work on that issue.
Diabetes has got to do with being "sweet": always putting others first, not being sweet to and for yourself. This can become a family issue, meaning that ppl who have this tendency can become diabetic, others that deal with it differently, don't.
Not that black and white of course, but that's the main thing that's behind diabetics (many diseases have underlying issues like that).
Some of these diseases could possibly be prevented before it manifests itself or in a very very early stage. However, if the disease has as good as fully manifested itself, it might not be possible anymore. Sometimes there's damage to the body, i.e. the pancreas not working anymore, which simply cannot be healed.
I'd recommend finding a really good reader/healer who can deal with this type of thing --> work on cellular / DNA level / cell memory and karmic (family) diseases.
If there is such an underlying issue at hand, and you manage to heal it, you can in actual fact heal the entire family line, meaning future generation may not become diabetic anymore.
I believe there are crystals that can help stabilize blood-sugar levels, but can't find them in my book right now. Maybe someone else can help you out with that.
Good luck!