On father demise you and your brother are legal heirs
2) I presume your mother predeceased your father
3) nominee is trustee for legal heirs
4) apply to society for transfer of flat in name of legal heirs . Enclose father death certificate , family certificate , indemnity bond in favour of society
5) if society insists you will have to apply for and obtain letters of administration in name of legal heirs
6) apply to court to being appointed as guardian for your brother