Your fact summary does not make sense. If your father was truly the only owner of the property, there would be no legal way for your sister to enter into a contract to sell the property.
If your father was the sole owner, the property would need to pass through probate. In the absence of a Will, the state's intestate code would apply, and presumably, your rights in the home would be the same as your sister's.
That suggests to me that she may have somehow been added to the title of the property. This could have been done either during your father's lifetime or afterwards, by way of a quit claim deed. You should contact the register of deeds office in the county where the property is located, or run an online check of the title of the property. You should be able to get a copy of any transfer documents, if your father DID add your sister's name to the property. You would then need to decide whether or not to challenge the transfer, on the basis of lack of capacity or undue influence.
Best of luck to you!