You should consult with an attorney in your father's area who is familiar with guardianships. Some states will honor a medical POA over the guardian's judgment. You should check to see what powers a guardian would have in light of your father's POA and medical directive. Good luck.
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My comments are not intended to establish an attorney-client relationship, are not confidential, and are not intended to constitute legal advice. Proper legal advice can only be given by an attorney who agrees to represent you, who reviews the facts of your specific case, who does not have a conflict of interest preventing the representation, and who is licensed as an attorney in the state where the law applies.
If your aunt is listed as first on the medical poa then she legally can make all medical decisions for your father. Even though you are his only child that does not give you the right to make health care decisions or financial decisions without either a valid power of attorney or a court appointed guardian.
You should discuss this situation with a local attorney who can best advise you as to how to proceed.