I also recommend having her thyroid checked as that can be a cause of weight gain as well.
Yes, unfortunately, 8.8 (158) is much too high for a fasting blood sugar. If she has put on a lot of weight, she might in fact have Type 2 diabetes which has become much more common in children. But only your doctor can diagnose diabetes, and tell you which type it is. I suggest you have her see her doctor as soon as possible.