She can't remember her PIN so keeps it on a piece of paper with her card which is obviously a very bad idea. She usually pays contactless but occasionally it asks for the PIN. I have mums banking app on my phone so I can see all transactions. She asked me to get her £100 yesterday afternoon from the ATM and by the evening she told me it had all gone and she didn't know where it is. I know he's stolen it but can't prove it. The same happened last Thurs her purse with £100 in it disappeared and never turned up. Absolute nightmare for me. The problem is mum thinks she can trust him as she can't remember everything he done in the past. I could change her PIN but she'll give it to him again.
Oh dear , this must be worrying for you
Would your mum go with you to the bank and agree to you being an appointee on the account? It would give you permission to act on the account .
Or... could you persuade her to move any money in the account that is more than is needed for her daily expenses. You could encourage her to open a saving account with interest that needs a passbook. It would make it harder to withdraw without her signature ,you could keep book for safe keeping if she agrees.
Or.. if you are appointee you can use the app to move money to a saving account or saving pot attached to the account. While it's in the saving pot I don't think you can withdraw on a card.
Does your brother have access to the bank app as well ?
Apply for deputyship if she has lost capacity.
Not sure what else to suggest that you can do.