Mutation of descendants names after death of mother died intestate

Sir/Madam , My mother died without a will but had nominated my elder sister in the society share certificate. Hence after her death the flat was transferred in her name in 2004 by the society after obtaining affidavit and NOC from myself (son) brother , Father. Now later in 2011 unmarried sister died without making a will and had nominated in favour of Eldest married sister and her son(nephew) ,hence the flat, society registration certificates was transferred in their name as previously with affidavit of other family members like myself and father. However after mothers death the mutation of name of elder expired sister had not been brought on record of sub registrar. Now Eldest sisters and Nephews name also has not been brought on record of sub registrar .Now Eldest sister and nephew wish to transfer the flat in my name or fathers name. How do we go about registering my name with the sub registrar in borivali west Mumbai. Thanks and best records