Your power in the power of attorney ended when your mother died. And banks have slightly different policies about who they will allow to get access to a deceased person's
If the bank has no separate authority to give you access -- for example, your name with on the account as a joint owned or you are listed as an authorized signator -- then many of them will allow only an executor of a will to access the accounts. To accomplish this, the executor must present bank officials with a formal legal document usually called Letters Testamentary that the probate court issues.
Other banks will allow access if you are able to present a notarized Affidavit of Assumption of Duties, which is a simple statement giving date of death, accompanied by a death certificate.
But if your real concern is that your sibling is siphoning the funds without any authorization, your best immediate bet may be to go to the bank authorities and inform them of the death, presenting a death certificate. This will effectively freeze the account until proper access and ownership can be straightened out.