@Spendysis you're misunderstanding how POA works.
POA doesn't get registered on a bank account, it is registered for a person.
So your sister has POA for finances (and health?) but is not using that authority to gain consent from the bank to access DM money, because DM has given permission for your sister to do so and also given access to bank card, PIN number, online banking password etc. Hence the bank being unaware of the POA.
Your sister hasn't needed to invoke her POA to access the money - because either:
DM has been manipulated into handing over her details and money,
OR
DM willingly did this because she favours your sister and is happy for her to have all her money as a sort of early inheritance, before she's actually dead.
Depends how you look upon it.
OPG/SS are looking upon it that whatever permissions has been given in the past, DM is currently mentally incapable of giving permission in the present day. Meaning that whether your sister needed to force access to bank accounts by using POA document or not, the fact remains that due to DMs mental incompetence your sister is currently legally acting under POA laws. Except - she isn't. She's abusing those laws and therefore by definition she's financially abusing DM. This is because sister is not using the money for DMs best interests, she's using it for her own best interests, which is illegal because it's not her money. This is what OPG/SS are seeking to stop by removing your sister from the POA document and transferring POA to DM's SW/SS dept (I'm not entirely sure of the exact details of how it works when the state has control over someone's finances).
You've done nothing wrong and you haven't misinterpreted instructions from OPG. They've suggested that you inform the bank and the care home what's going on, so that both those organisations can implement whatever protective measures they have available for preventing their clients from abuse/fraud. Because your sister isn't likely to inform them herself that she's under investigation for those crimes. So if nobody tells them, they won't know there's a problem. Your DM social worker could also have done this, but obviously that adds to their workload and OPG knows you're invested in your DM welfare otherwise you wouldn't have even bothered bringing what your sister was doing to their attention and trying to get her stopped. So they've naturally assumed you might be happy to help by doing this other task of informing bank and care home.
So you've discovered the bank didn't know about your sister having POA and you've told them. This is good because now they know. Now they're raising their own internal fraud investigations, I would think. Because that's what it is when someone is wrongly accessing someone else's bank account.
The fact your sister hasn't informed them of the POA and is accessing the accounts on the sly, initially with DM permission, won't go in her favour. Nor will it look good on her that OPG are so concerned with her behaviour around the use of the money, that they (OPG) have decided she's not complying with the POA laws and are seeking to remove that power from her.
I wouldn't be surprised if the bank freeze DM accounts until this is sorted, as a fraud prevention measure.
They also need to establish what fraud has already occurred, if any, (it has but they can't take your word for it they need evidence, so they're investigating).
Then the bank will need to work out if they'd taken all the measures they could to prevent fraud, or if they didn't do that and they're liable to refund DM some of the money (that's been stolen by your sister) themselves. I don't think they'll be liable for this because DM initially gave your sister permission and means to access the accounts, when she did have mental capacity to do so.
The bank may also have to see if they can recover any of the money that's been stolen by your sister and return it to DM's account. I've no idea really if they'll have any luck with this. I don't think they'll get anything back from your sister because she hasn't got a pot to piss in and has spent any money she took. Whether they can get it back from any shops or businesses it's been spent at, I don't know. That probably comes down to whether it can be proved that these purchases were purely for your sister's benefit or not, and obviously whether or not there's a paper trail of the money going to X or Y place. If she took it all out as cash and nobody knows where it's gone then I don't think the bank will be able to recover any of it.
It's the bank's duty to help their client if the client has been defrauded and to help prevent fraud occurring in the first place, that's why they're looking into all this and asking you questions as part of their investigation.