My deceased father's estate

A power of attorney has no legal effect when the person granting it has died.  This is because the agent under a power of attorney cannot take directions of, act of behalf of a report to a dead person. When someone dies without a Will, that person's property passes to his heirs under the state's laws of heirship, whether by court declaration or by an affidavit of heirship filed in the deed records.  This last will not be enforced by a court until it has been of record with no one complaining that they were omitted. Check the records of the local probate court and check the county deed records to see whether your sister had any apparent authority to sell the house.  Then hire a local probate lawyer to recover your interest in your father's estate.