the property will have to be mutated to include the names of yourself, your sister and your mother alongwith your father
then after you obtain the probate of your father's Will, the PR Card will again have to be mutated to delete the name of your father and insert your mother's name having 50% share and balance 50% being held by the 2 children
you can make a MOU in the meanwhile with the developers before signing the DA
Under the MOU it can be agreed that condition precedent for signing the DA would be that the grantors of development rights have to take steps for mutating the PR card to reflect the correct owners as per the grandmother's and father's Will
so until the condition precedent in the MOU as aforesaid is not complied, the developer will not get any development rights. But that's the choice of the developer if he is ready to wait for the father's Will being probated and the PRC being mutated to reflect the correct land owners. The developer is exercising his due diligence. Some developers are a bit adventurous and directly enter into a DA despite pending compliance by the land owner, by taking an indemnity from the land owner. But given how matters get blocked in the courts in case of any dispute claim or litigation, many developers are very cautious and want all legal compliance to be in place before signing the dotted line in the actual contract