Disclaimer: The materials provide below are informational and should not be relied upon as legal advice.
I am not clear on your question. Who has stopped you from visiting your great grandmother? What were you told when you were refused visitation? A power of attorney is not used to prevent people from visiting a sick person.
My colleague is correct, this would not be a correct use of a power of attorney. He has no power to instruct hospice not to permit visits.
The above is general legal and business analysis. It is not "legal advise" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
A health care power of attorney can be used to make medical decisions on behalf of a person. However, preventing family visits is highly irregular, and an emergency petition to your local probate judge would likely permit family visitation. A lawyer letter demanding reasonable visitation would likely have the same effect. A regular financial power of attorney should have no effect over visitation.
The answers provided are for general information purposes, and are not legal advice. By providing this general information, we are not representing the questioner or any other reader. Do not rely on this advice as personal legal advice.