Transfer of Flat to legal heirs in case of death

Dear Sir/madam, My grandfather owns one flat in a housing society in which I am currently staying with my family. He died in a year 2010. He is having two sons including my father and 2 daughters. He has written a proper will in which he transferred the above flat to my father alone. My father was an associate member in a society and my grand father also nominate him for the said flat. After death of my grand father society has transferred the flat in the name of my father on the basis of above documents. Now our building is under redevelopment from last one year and new agreement has been made with the builder.Now my uncle i.e. eldest son of my grandfather sent a notice through an advocate to our society stating that as per bye laws, society cannot transfer the flat to my father without taking NOC/consent of the other legal heir (His). He also asked society to provide copy of all documents on basis the said property is transferred to my father and if not provided then he will take the matter to the registrar of society.Kindly suggest what we suppose to do in such a situation? whether my father is legally owned the said flat? whether a will needs to be probated? Please advise.