1. Assuming that your father died intestate ( without executing a WILL ), then the share in the property devolves equally to his Class-1 legal heirs.
2. In the instant case, the share in the property gets devolved equally to your mother, your sisters and you.
3. Since your mother and sisters have recognised your efforts in constructing first and second floors and are ready to give up their individual share in your favour, your mother and sisters have to execute a registered Release/Relinquishment Deed in your favour in the jurisdictional Sub Registrar's Office to make you sole owner of the property.
4. You have to produce your father's death certificate, family tree, property documents, tax paid certificate, patta of the land, etc.