I am sorry your father passed away. I am assuming your father was not married. You have all the rights as an heir at law, she has none. If she is named a beneficiary under his will, or personal representative, she has rights. You need to hire a probate attorney NOW.
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I am not sure how or on what document she is listed as next of kin. If it was in hospital documents it would not have any legal significance. She would need to be listed as a beneficiary in a will or trust or some other document which provided her legal rights for her benefit from such a designation.
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.
I am sorry for the loss of your father. Unless your father had a will that specifically named his girlfriend in it, and provided for her, then she would have no legal right to any part of his estate, except property that she jointly owned with him (a car in both parites names, a home in both parties name etc..). You should consider hiring an attorney to help you understand and probate his estate right away.
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