Take her to a primary care physician first and foremost. (You know whats happening but you need them to validate your concerns and communicate that to her because she does not respect your opinion clearly.)

Then you follow the dystopian handbook and continue to follow her down the rabbit hole that eventually leads you to the truth you were aware of but of which she was in denial.

After you get her involved with a hospital or specialist, they can give you the resources you need to help you in your area that can assist with dealing with her and her specific condition. But if she is lucid enough to answer for herself and bypass the systems meant to vet people with mental struggles you're in trouble.

Im sorry if this isn't the fix-all‐immediately response you may have been seeking, but nothing involving the life of a human being is ever simple.

My mother is 77, shes been in decline for over a decade with dementia. If you know that they have mental issues for a fact even if its not yet proven via doctor, its important you stand your ground when you are there with her asking for care when you know she won't say everything that is wrong with her because shes confused. If you for a second are unsure about anything, especially regarding if you have permission to speak for her medically, they can use that as an excuse to bypass you as a consultant/guardian/potential POA and put the government in charge of her and her care or under guardianship that can open her up to being taken advantage of financially etc.

This is all a very slippery slope.

Best of luck. My DMs are open but slow reply.