My brother was poa for my mom and racked up considerable amount…

You are not required to give the credit card company or collection agency any documentation. You do not need to cooperate with them in anyway or respond to their calls or letters. They do not have any right to demand or receive a copy of the power of attorney that you now have, unless you are wanting to negotiate with them on behalf of your mother.

But there be no point to negotiating or having any discussions with them.

Just because you have a power of attorney for your mother does not make you legally responsible for her debts in any way. The collection agency cannot enforce against you personally. And if she has only income from a pension and no significant assets, then even if they sue her and get a judgment she is essentially judgment proof. They have no way to enforce the judgment. Also, if the debt is more than two years old and there have been no partial payments or formal acknowledgment by the creditor that the debt still exists in that time, then they cannot sue. They would be outside of a limitation period to do so. The debt still exists and they could pester you for payment, which you could ignore, as they have no way to sue in that case.

You can tell the collection agency that you will not respond further in any way to their calls or letters and will complain to the Ontario government if they continue to call or harass you.

I hope this is helpful. If you have no further questions, please rate the answer positively, as otherwise the site does not credit me, even if you have paid a deposit. You can still ask follow up questions after rating though, if you wish.