Can you have snoop around when you next visit mum, and see if there are any bank letters about? I don't know how well a bank would take a "does this person bank here?" query, but maybe that's my suspicious mind! Does your mum still have bank cards, even though she no longer uses them?
To be honest, I think your mum would be best off in care. Even you being much nearer wouldn't be 24hr care, which I suspect is what she really needs. You would also be very tied to your mum, and might find it difficult to balance work and her needs. However, trying to persuade mum and brother might be tricky. I would be inclined to wait for SS assessment, as this will give you ammunition if your brother becomes difficult. He can't say you are insisting on care that mum doesn't need.