Yes, there's a target HbA1C before you get pregnant. This varies slightly from doctor to doctor. But is usually something like a max of 7.5 or less.
In pregnancy, the target is basically to keep your blood sugar as tightly controlled as possible. My last HbA1C in pregnancy was 4.7.
The (hopefully) low HbA1C comes from keeping your blood sugar very tightly controlled. Your sister should be given targets eg less than 6.0 before meals and less than 7.8 two hours after meals. So in pregnancy 4s and 5s are ideal, with 6s and 7s after meals. These are very strict targets and take a lot of work
Please feel free to ask any more questions