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I would suggest that you find someone outside of the family, say a clergy person or a family friend she trusts, and have them talk with her. Often, non-family members get farther, as the family dynamics are removed.
You also should have her doctor evaluate her for dementia. If she is positive, or if she just has some age related memory issues, the doctor may be able to convince her to give up the checkbook.
This is always touchy territory. Good luck. It won't be a walk in the park.