Type 1 Diabetes is not related to diet. Nothing you eat will make this disease develop.
Also decreasing insulin is not the goal for a Type 1 diabetic as their body does not produce it anymore and they WILL DIE without it. However, controlling her blood glucose numbers is what is keeping your mother from needing more insulin. if her blood glucose numbers were higher or she was eating higher grams of carbohydrates then she would NEED more insulin.
As for the op, i would recommend you speak with your doctor and find out what they want you to do about her "pre" diabetes control. Also what type of diabetes do they think she is predisposed to? I am assumming Type 2 as you cannot stop type 1 from happening with diet.