Yes, my mother has decided to take legal advice. She will be asking aunt not to take any further action whatsoever until she gets back to her after having spoken to a solicitor. I suppose the next step will be to go to the bank and ask them to freeze A's account until a third party can be entrusted with her financial affairs.
I really appreciated all of the responses as they've made me try to see the other side. I can see how the aunt might feel that my mother is trying to do the same thing -- take control and make unilateral decisions.
We would like to do something that doesn't completely ruin the family and personal relationships involved, but a huge amount of damage has unfortunately already been done. We'll see what we can salvage. Thanks again for reading and for the feedback.