What should I tell my defiant mother that she needs to get evaluated with a neurologist?

My Father has Alzheimer's, and has had several strokes, so he is memory impaired. He also has had diabetes for 25 yrs. He is easy to care for. My mother is the main caregiver, although my sister and I help a lot. the last couple of years my mother's personality has changed, she is defiant to any help, or abiding by doctors rules, she doesn't want any healthcare devices in her house to help my father. she still drives, but is starting to lose her direction. Me and my sister or our spouses do the driving as much as we can. he is confused about a lot of things most days. Even her mail. Her bills are now auto-pay. But some documents that come in the mail are not understood. I want to have her checked for dementia but I don't know how to approach it. I started taking my father to a neurologist just recently to have him reassessed. She doesn't see the point. She wants to quit giving him the patches that were prescribed for him just 3 weeks ago because they cost too much and they don't help. She is giving him sweets for snacks on a daily basis. Like doughnuts and coffee, chocolate chip cookies and coffee, ice cream at 9pm. Im surprised that daddy hasn't had anything cut off although his eyes have popping blood vessels and is almost blind. They are both 85 yrs old. I would appreciate any ideas. I may ask the neurologist to tell her that the caregivers need evaluation.