Mom's mind is slipping, and doesn't want any help. I would appreciate advice or support.

My mother is 75 and has Parkinson's. She's had it for about 7 years, and her mind is really starting to fail, to the point that she no longer remembers her medicine, when I will come to her home and call her (I come every other day and call every morning at 7 AM), stumbles to remember words to common objects and forgets how to operate the telephone and television. Physically though she is in excellent shape (except for the Parkinson's tremor). Her balance is great, and although her walking is slow, she gets around the house well and is still able to dress herself, use the facilities, and even make the bed. When I come over I do her cleaning, cook her fresh meals, and do her gardening and bills. I take her to the store and out for breakfast. Everything has been going fine, but I've seen a huge downturn in her mind lately. She is becoming more and more confused and it alarms me. I have brought up trying to get extra help (I am the only family in the area), and she keeps saying "we'll see." When I brought up how worried I am about her and her mind slipping, she said "I thought I was doing great!" My home is not safe for her to live in (and after caring for my severely disabled father who passed away two years ago, I am mentally burned out), and I just don't know what to do. My brother lives across the country and has told me he thinks I am doing everything I can since legally I can't force her to accept nursing help or go to an assisted living facility. He said I should just let her be and keep helping her how I am. But I worry about her all of the time. After a recent visit to the ER for a UTI, the ER doctor told me she shouldn't be alone because she was dehydrated, and she probably was forgetting to stay on top of drinking water. She's also lost 6 pounds in the past 6 months, and that might not sound like much, but she is only 95 pounds (5 feet tall). I would very much appreciate any advice or support. I just feel hopeless.