Gestational diabetes is also diagnosed based on plasma glucose values measured during the OGTT. Blood glucose levels are checked four times during the test. If your blood glucose levels are above normal at least twice during the test, you have gestational diabetes. Below shows the results for the OGTT for gestational diabetes.
Fasting: 95 mg/dL or higher
At 1 hour: 180 mg/dL or higher
At 2 hours: 155 mg/dL or higher
At 3 hours: 140 mg/dL or higher
So your sister does not have Gestational diabetes.
If you want to know details about gestational diabetes, Visit:
http://healthy-ojas.com/html/diabetes_mellitus.html