In general, a fiduciary is not allowed to take actions from which he or she would benefit. Creating a joint account with a right of survivorship benefits the fiduciary. It would be permissible if the POA specifically allowed it, impermissible if it did not.
You have stumbled upon a backhanded way to correct the fact that your bank allowed the apparent change in account ownership. Tell the attorney-in-fact that the owner must agree to the change; i.e. sign the signature card. (For there to be a right of survivorship in Texas, the card has to be signed by the person who died.)
Do not give the attorney-in-fact the card in order to obtain the owner's signature. Have the owner come in or send a bank employee to see the owner and make certain that joint ownership is what the owner wants and that he understands it is not necessary if all he wants is someone to sign on his account.
I respect Brenda C; she's the kind of person I would give a POA. Nevertheless, I would not give her the power to convert my property to her property.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.