Given your mother's change in ability to do something as basic as write a check, and the fact her sister already has Alzheimer's, I suggest you make an appointment with
Diabetes does affect the brain. Periods of high or low glucose make it difficult for the brain to function. The brain needs at least 130 grams of carbohydrate per day (and not all at once!) for optimal function. With diabetes, carbohydrate counting is key to minimizing glucose fluctuations. Some people interpret this to mean cutting down or cutting out carbs. But in this case, less is not more if less equals fewer than 130 grams of carbohydrate. So one simple thing you can do right away is check to see if your mother's carbohydrate intake is adequate. Ideally, women can have 45 grams of carbohydrate for meals and 20 grams of carb for snacks. An example meal could be a sandwich and an apple, and for a snack, a small yogurt.
That said, diabetes is also a known risk factor for Alzheimer's disease. Excessive blood levels of insulin, which occurs in type 2 diabetes, may damage the brain in ways that can make a person more susceptible to Alzheimer's disease. Other risk factors that may contribute to your mother's recent memory loss, such as high blood pressure and excess weight, are also commonly seen with diabetes. If she does have the beginning stages of Alzheimer's, her doctor can suggest treatment options, including medications, exercise -- both physical and mental -- and dietary changes that may help your mother maintain her brain function and memory.
her primary care doctor and, if you can, go with her to her share your observations. Ask to have your mom professionally evaluated for this brain disorder.