Hi from just answer. I'mCustomer I will try to assist.
First, I'm sorry for you and these issues. I went through this myself in caring for a parent.
Second, social security assumes people can manage their own affairs until they indicate they can not. So, she can draw on accounts, change them, or take any steps she wishes until they recognize her condition or you act to intervene.
You can get yourself appointed as a representative payee, which would give you control over her account and benefits. But, if you have a dispute with her or others about how best to manage, I suggest that you find a way to make the peace with all involved to fix things.
Your alternative may be legal action in which you may have to prove she is incompetent and that you must be appointed as per of attorney over her affairs. This is costly, not just for the legal talent involved, but for the emotional cost. For everyone.
I suggest that you meet with the children, even going so far as to offer them a review of how her benefits have been spent over the last 6 months or so, to prove there is nothing unseemly for them to be concerned about.