When someone gets purple discolouration of fingers then it is either due to a blunt trauma
or due to blunted blood supply to the fingers. If you are sure that her fingers were never traumatised then please take her to a vascular surgeon to make sure the blood supply is put back in place by first identifying the reason of this and then treating it on time. Do not delay as stagnant blood in the fingers can lead to infection and further problems. Sugars are not related. This can be a vascular problem for sure. Take care.