1) apply for letters of administration from district court in the name of mother and your self
2) enclose father death certificate
3) you have not mentioned whether your brother was married or not
4) if he was married his wife and children would also be the legal heirs as they would inherit your brother share in property
5) letter of administration does not take more than 6 months
6) on obtaining LA apply to society for transfer of flat in the name of legal heirs