A power of attorney is meant to take legal effect when a person becomes incapacitated and can't make decisions for themselves - so, once someone is incapacitated, they are unable to make decisions and so don't have the capacity to sign valid legal documents such as a power of attorney. I recommend first making sure that your aunt didn't execute a power of attorney previously - check with her attorney, in her files and safe deposit box. If not, medical decisions can be made by family members and there probably is a law in NH regarding priority. Usually, spouses have priority, then adult children, then parents, then siblings, etc. The problem is that if there is more than one person in the prioritized group that gets to decide - for instance, 3 adult children - they all have to agree. If you can't get agreement, you can try to get an emergency appointment as guardian for your aunt from the probate court.
The exact terms of the guardianship agreement, and N.D. law on a guardiann's duties and how co-guardianships are conducted will govern the outcome in this instance.
I recommend you consult with a guardianship attorney. You will not be able to obtain a (durable) power of attorney for someone who is mentally incapacitated. As the other attorney indicated in her answer, you should check to see if there is an existing durable power of attorney for health care (old NH law) or advance directive (new NH law). If there is such a document, the agent named under the document can act with respect to the medical affairs of the principal. If there is not such a document, you can petition the applicable probate court to establish a guardianship over the person. The guardian will be able to act with respect to the medical affairs of the person.
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