Regardless of marital status, each of us has the right to choose the agent for our Healthcare Power of Attorney. One's position as spouse does not give a wife the right to make medical decisions for her husband, nor does it allow her to override a valid Healthcare Power of Attorney.
If for some reason the POA is not found to be valid, a wife typically still does not have the right to make healthcare decisions for her husband, or even to find out his medical status in the hospital.
Because of HIPAA, the brother or sister also needs to have a Universal HIPAA Release so that if brother ends up in the hospital his named medical agents have the ability to call and check on him, access his medical records in order to make decisions, etc. Many people wrongly assume that if you are named as agent (or have a blood or marriage relationship) you will have access to the relevant information. Not so under HIPAA. Medical providers are increasingly afraid of being sued for violating HIPAA, and often hesitate to give out any information to anyone, regardless of relationship.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.