The Squire is right.
Just because a person's fasting level looks ok, doesn't mean that they don't have severe blood glucose fluctuations after food.
I should have a word with your sister and get her to ask for an Hba1c test.
In addition, just because a fasting test looks ok, doesn't prove very much at all. My fasting tests vary hugely, depending on time of day, stress level and what I ate the night before, ranging from 5.5. (Normal) to 8.5 (definitely diabetic!).