1) the seller ought to have executed regd sale deed in favour of your father
2) in order to confer clear and marketable title to the property regsitered instrument is mandatory .
3) section 54 of transfer of property provides as under
54. “Sale” defined.—‘‘Sale” is a transfer of ownership in exchange for a price paid or promised or part-paid and part-promised. Sale how made.—3Such transfer, in the case of tangible immoveable property of the value of one hundred rupees and upwards, or in the case of a reversion or other intangible thing, can be made only by a registered instrument. 1 Contract for sale.—A contract for the sale of immoveable property is a contract that a sale of such property shall take place on terms settled between the parties. It does not, of itself, create any interest in or charge on such property
4) is there a clause in your agreement for sale that the sale deed shall be executed at the convenience of the purchaser, as and when she wants them to be executed either in her name or in the name of her nominee or nominees.
5) if seller had failed to execute sale deed your father ought to have filed suit for specific performance within period of 3 years