You will not be able to have any control over the account as your sister is the appointee of the account.
The Department of Work and Pensions will only remove an appointee for good reason such as abuse of your Mum's funds.
In order to get reimbursed for any items bought for your Mum you will need to give the receipts to your sister.
If there are any further points please reply I will be happy to respond.
you can request statements but not demand them.
The whole point of having an appointee , attorney or deputy is that there is one point of focus and one letter can be sent.
You can apply to be deputy but only if there is lack of mental capacity when any appointment ceases to have any effect