Telling your parent she is no longer in charge of her finances is always a difficult subject. There are good approaches that work. First, the person who is most favored,
closest to her, or most trusted should be the one to broach the subject. If your family members do not feel up to the task, enlist the help of her attorney, if she has one, as that person can explain the power of attorney document,and may have prepared it. The parent's physician, if available, can be "the bad guy" if needed, to advise mom that she has changes to her brain that make this necessary from a medical point of view. If you are going about this on your own, I have written a booklet on how to take over management of an elder's money, which talks about this in more detail, and gives you further specific advice. It's called How to Handle Money for Aging Loved Ones, and it's available in downloadable format, or in print at AgingParents.com, my website, or on Amazon.