A thorough evaluation by a geriatric psychiatrist would be advisable. At that point, have a clear understanding and prognosis of her condition.Oftentimes, the elderly may be suffering from depression that may mimic dementia. If this is not the case and she is truly has a sole diagnosis of dementia,
family members including yourself would need to make a decision if you would want to petition to the court and declare her incompetent and based on the merits of your psychiatrist findings and her presentation, the court can grant you (and the rest of family members) limited or unlimited custodial care of her.
I advise tat you consult a legal counsel specializing in family courts as well.