1) did your father fill any nomination form for his 50 per cent share in flat ?
2)since your father died intestate legal heirs should apply for letters of administration from Bombay high court . you and your brother can execute consent affidavit .court would issue LA in mothers name
3) it takes around 6 months
4) then apply for transfer of flat in name of mother
5) if your father has made your mother nominee of flat society would transfer flat in mother name on basis of the nomination . you and your brother can execute gift deed or relinquishment deed for your share in flat