I used to be a geriatric care manager working with people who have dementia, in the DC area, and now i'm a mental health worker in a rural county. I'm a social worker. I have a couple of thoughts. And as they say in AA, take what you like and leave the rest.
Her judgment was definitely off about not wanting to bother people as she got sicker and sicker. Her behavior may come from low self esteem in general, depression, or minimizing her illness, and i know a lot of young adults and middle aged people who do that. A thorough assessment is a good idea in general, and if you can be there, that would be helpful. I wouldn't jump to dementia and therefore incompetence, guardianship, power of attorney for medical decisions and financial ones... but eventually she's going to need to take care of her affairs and designate decision makers anyway. There's a lot that can be done to help her be safe right now. And you can be gentle and gradual about conversations regarding what she would want done if...
I don't know her financial assets or circumstances but there are a lot of options for helping her stay connected to people and activity out in the world. Are there activities that she used to do that she's not doing much now? Golfing, thrift shop expeditions, church or temple, getting her hair done, going to movies, knitting or other crafts? Can she get back to the things she loves to do?
There's driving or other transportation to check on. There's safety in the home: does she need an alert button? do members of her family need to take turns calling her every day to make sure she's okay? maybe meals on wheels would give her some added nutrition, and if the delivery person doesn't get an answer, there's a protocol they follow. Does a neighbor have a key, in case she's not answering and you're worried? Does she have a cell phone? Does she keep it on her body (in case she falls)? Are there throw rugs that need to get removed so she doesn't fall?
You can find the area agency on aging by putting in her zip code in www.eldercare.gov and you or her sister can go visit with a social worker there to see what services are available that are free or subsidized.
It would be great for all of us on this site to hear from you later as you learn stuff and find out what is helpful. Feel free to leave more detail if you want, too. Astraea is right, that is a great book.
I'd make sure she isn't depressed, and make sure she isn't missing opportunities to stay physically and socially active. She will decline much faster if she doesn't move and doesn't talk to people regularly. Use it or lose it as they say.
Thanks for writing. Please write more. Hope some of this is helpful.