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We understand your concerns]
I went through your problem description. I suggest you not to worry too much. Your mother is already 79 and must be under some sort of treatment. Normally people of old age do have some sort of delusions which comes out of the anxiety
feeling. Happens when they see the reality.
I sincerely think that your mother is going through such a period. From my experience, I can advice you to ignore it. If you start advising your mother, it is of no use. The advises goes to dumb years, because she is preoccupied. Therefore, leave it to her wisdom. Just assist her.
Hope this clears your doubts. Available for further clarifications.