Does your mum still have mental capacity and would she understand the significance of adding your brother as a joint account holder?
The bank may not even speak to you about those accounts unless and until you lodge the POA(s) with them.
Even then, assuming your mum had capacity, the decision to add your brother to the accounts may be accepted as a perfectly valid decision for her to have taken.
Is the POA joint and several - ie can you make decisions independently - or joint - ie all attorneys have to make decisions together?
Speak to the bank, certainly, about this issue and ask about registering the POA with them.
Thereafter, ask if there's any way the decision to add your brother to the account could be re-visited as the intention seems to have been to enable access to the account for the payment of bills and a general overseeing of your mother's finances. Not for him to become joint account holder (and possibly sole beneficiary further down the line).
Forgive me, but do you think this was a deliberate ploy by your brother, or has he (and your mum) simply acted naively in the honest but mistaken belief that they're doing the right thing?
Does he understand how the POA operates?
Also, could sister in law be meddling here? You say she's executor too.
Good luck.