Grandfathers property Sale

My grandfather died without leaving a will. My mother wants to sell the property and share it equally between all her siblings two of whom have died. Unfortunately, one of the grandsons (son of the deceased sibling) is not willing to sign the sale documents unless he is given a bigger share. I have been advised that all the children of the deceased siblings have to sign the sale deed so as not to cause an issue for the buyer in future. My mother has lived in the house with my grandparents throughout her life and has taken care of them as well as the house including payment of house taxes etc.The grandson is using the excuse that my mother lived in this house rent free and hence he needs a larger share as his dad (who is deceased) had to live separately .The truth is his dad deserted his parents and never came to even see them when they were alive because they were not happy with his choice of a life partner(the grandson's mother). Please advise if my understanding is correct regarding the signatures of all on the sale deed. What options does my mother have if the grandson prevents her from selling the property so she can share it equally between all siblings?One of the siblings is unmarried and lives with my mother in the same house. All of them are very advanced in age and my mother wishes to settle this matter before anything happens to her.