Transfer of ownership of flat in the absence of a will

My parents owned a flat in Mumbai in a cooperative housing society. When my father passed away, my mother's name was entered in the share certificate as the owner. My mother too has passed away now and she left no will. But my sister and I are nominees to the flat. We have no other siblings. Do I need to provide any documents to the society to get the share certificate transferred in the joint names of my sister and me?