Hi,
People on this forum aren't in a position to diagnose your sister's condition - only her medical advisers are able to help with that matter.
However, if she does have problems with elevated blood glucose levels then, in my opinion, the best thing that she can do is to cut back on the starchy carbohydrates i.e. cereals, bread, potatoes, pasta and rice etc. That usually leads to a dramatic and quick improvement.
As far as I am concerned, making such dietary changes is more effective than either weight loss or exercise. However, changes in these latter two areas will certainly help too.
It is quite possible to delay the onset of Type 2 diabetes by making such lifestyle changes.
Best wishes - John