Unfortunately, there is no process for your brother to add another person as backup attorney for your mother.
Under the law in BC, an adult can appoint an attorney who can make decisions on their behalf if the adult becomes mentally incapable. This is called an “enduring power of attorney”. The authority of an attorney ends if the attorney becomes incapable or dies. An adult who has made an enduring power of attorney can change it, unless he or she is "incapable of understanding the nature and consequences of doing so". That is, if the adult has lost their capacity, they cannot change their power of attorney.
There are other options you might want to consider.
A section 7 representation agreement
Someone who doesn’t have the legal capacity to make or change an enduring power of attorney may still be able to make a representation agreement. A person can make a “section 7 representation agreement” even if they cannot manage their routine financial affairs or look after their daily needs. A representation agreement is another type of legal document that enables an adult to authorize someone to make decisions for them when they can no longer manage on their own. The “representative” can make decisions relating to health care and personal care matters. With a “section 7 representation agreement”, the representative can also be authorized to handle “routine management” of financial affairs and most legal matters. To learn more about representation agreements, see the Nidus website.
A pension trusteeship
If the older adult's financial situation is not complicated (for example, there are no significant assets and their only income is federal pension monies), a "pension trusteeship" with the income security programs in Ottawa could be sufficient. This option is more formally known as a "third party administrator" of pension funds. A person, organization or agency can apply to be the administrator of the adult's pension funds. In Vancouver, the Bloom Group is an agency that provides this service for incapable people, including homeless individuals. See this page for more information: Supporting Homeless Seniors - Third party administrators (although this page speaks of supporting homeless seniors, this program is not limited to homeless seniors).
If an adult is mentally incapable of managing their own affairs, someone can be appointed to manage the adult's affairs on their behalf. This is referred to as "committeeship". A committee must apply and be appointed by the BC Supreme Court under the Patients Property Act. For information on this process, see this information on committeeship from the Canadian Bar Association of BC.
Public Guardian as committee
The Public Guardian and Trustee could become the committee or "statutory property guardian" if no one else is available or willing. The Public Guardian does charge a fee for its services. They should be contacted for more information.