Property inheritance as property was registered in name of grandmother

We have about a 1000 sft flat at Cumbala hill Mumbai. The flat belonged to my grandfather. My grandfather was on rent in 1960s of 2 flats, one this at Cumbala hill and other at Malabar hills and vacated his rental agreement. He transpired the flat at Malabar hill to his step daughter and the other flat to his wife which came in name of my grandfather. My grandmother nominated my father and Uncle in Housing society. Now upon death of my grandmother in 2004 and my Uncle in 2007. My grandfather already expired in 2000. My uncle having no family and only one blood brother. The step daughter was daughter born to my grandmother from her first wedlock. She acquired lots of property from her forefathers. This lady step daughter is claiming share of 1/3 from her mother's portion (who already nominated 2 son's) in 2015. The nomination was filed in society and society transferred the suit in name of my father as owner. Is this case of step daughter valid as she is challenging nomination.