CHS Flat transfer when nomination and WILL are in place

I belong to a Hindu family and jointly purchased a flat with my mother in a Mumbai CHS in 1990. Along with being a joint owner, I am also the sole nominee in the said flat. My mother has passed away bequeathing the flat to me through a registered WILL. I have a lone sibling, a married sister, who is challenging the WILL and trying to stop the transfer of the flat's shares in my name. Need clear and simple remedies to transfer the said flat in my name asap. Thanking all the esteemed counsels in advance.