Probably late to the party here, but this might be of assistance. Banks tend to use terminology like "signatory" and "authorising party". A signatory can make transactions on the account (ie withdrawals), but it takes an authorising party to make changes to the structure of the account - like who can be a signatory. Two very different animals.
I would consider this angle and see if you can get FOS to relook at it: Does the POA allow the attorneys to act as authorising party on the account, or simply signatories? If the latter, then, did the bank make the decision to remove you as signatory because you mother had lost capacity? In any case, it's a joint POA - not joint and several - so your sibling cannot make the determination on her own.
Depending on where you are, another potential option is an administrative tribunal. For example, in Queensland the Queensland Civil and Administrative Tribunal (QCAT) can hear matters about adult guardianship. This would be an action directly against your sibling, and not the bank. You can read about their jurisdiction here:
Decision-making for adults - QCAT Queensland Civil and Administrative Tribunal. Your mileage in other states may vary.