Transfer of Flat Ownership Upon Death of Member and No Nomination done

Dear Sir, My father owned a flat in a Society, before death he had nominated me in the Nomination Form as 100% nominee, but there was no date mentioned in the nomination Form. Now upon death of my father the society is not accepting or admitting the Nomination Form and not transferring the Flat in my name. I am the legal heir; also I have two (2) sisters who do not have any interest or stake in the Flat. Please advice and guide me what legal procedure is to be followed to get the Flat transferred in my name so that I can become the rightful owner. My father has also made a Will wherein he mentions that the said property belongs to me and his daughters have no claim on it. Please advice the course of action and the set of documents to make to get the property transferred in my name.