How can wife get power attorney from sibling for mother?

Typically anything written which indicates you are revoking it should be sufficient. It should be delivered to your agent. I would also deliver it to any banks or other places where your agent may use the POA.

I can assist with that

A POA is best done through an attorney to ensure the verifying/interested parties are proper, the agreement includes what the principal wants, the principal is competent in case of a challenge and it's filed properly if necessary, for example if it will be used to transfer property. All persons should have a POA, HCPOA, Advanced Directives and Will. You can also pull the POA form out of the general statutes of NC. The form is included in the law.

A power of attorney allows you to make decisions and dispose of property, it does sound like one might be helpful in your situation. When an attorney writes up the power of attorney, you have a range of permissions to cover all the needs of your individual situation. You’ll want to be sure that you have the ability to write checks, etc. Additionally, the power of attorney will be useful if other family members challenge your decisions or actions. Drafting the document is a fairly straightforward process, and most attorneys will bill 1-2 hours depending on whether you need additional documents for healthcare and estate decisions.

You need an attorney to review your case in order to provide you with an answer.