The narration of the case is very confusing and with due respect I say "how the learned lawyers are giving opinions and advice?"
Is the property a flat in a co-operative housing society? In which State is the society? Is it in Maharashtra? Who was the original owner of the flat? How did the mother acquire rights on the flat? Did she inherit it from her husband through a Will or otherwise? If she was sole member of the society with respect to the flat what kind of documents had she made to transfer the rights to her 4 sons in the manner as stated here. Had she executed a "will", or had she filed nomination with the society or was there a family settlement? If two of the brothers do not want any rights they have to execute only relinquishment deeds. It will involve only nominal stamp duty. There are Bombay High Court judgments in this regard. Section 30 of the Maharashtra Co-operative Societies Act, 1960 deals with the transfer of interest in the Capital and Property of the society. A society's interest is only with regard to the liabilities of a member to the society. The ownership will have to be decided if there are claimants or there is a dispute. Also when a property is to be alienated, the buyer may demand proof of ownership of the seller. The share certificate of co-operative society gives the names of all members if there are joint members. It does not give the proportion in which shares are held by each of the joint members. I am stating this with my experience in managing a co-operative housing society for the past 30 years.