What happens if my sister was left as my father's power of attorney?

Your question does not contain sufficient information to provide you with a complete answer. Is your father deceased? If yes, then his power of attorney became ineffective on the date of his death. If your father is still living, then (depending on the specific language contained within his power of attorney), your sister may have the ability to manage and make decisions regarding all of your father's property on your father's behalf - including real estate. To determine ownership of the property, you can contact the Register of Deeds for the county in which the real estate is located. If you own the property jointly with your siblings, you must come to an agreement together regarding who can reside in the property, how the expenses of the property are paid, and whether or not rent is collected. If you cannot come to an agreement, then a court proceeding may be required to resolve the dispute.