1. Your father is the co-owner of the property i.e. he owns 50% of the property.
2. After his demise, you can not sell off his share of the property without being owner of his said share of the property.
3. You shall be able to sell your father's share of the property if he has executed a will bequeathing his said share of the property in your name after taking probate of the will from the Court.
4. If your only sister registers a deed relinquishing her share on your father's share of the property, you become absolute owner of the entire property can sell it off.
5. Alternatively, if your father registers a gift deed in your favour now duly conveying his share of the property to you, you become its absolute owner for being entitled to sell it off. Your signature will not be required by your father for executing and registering the said gift deed.