Can biological mother write will on behalf of her mentally retarded son

My maternal uncle is mentally retarded and he has property on his name. My maternal grandmother is alive and maternal grandfather expired. My mother is taking care of mentally retarded maternal uncle and maternal grandmother from last 20 years. Now my grandmother want to write a will of her property and on behalf of my maternal uncle property. Can a biological mother write a will against her mentally retarded son. My grandmother want to give her and her mentally retarded son property to my mother. What is the right way to proceed. My grandmother has 3 daughters and one mentally retarded son. Of the three daughters, elder daughter expired, my mother is second daughter, and third daughter is alive and lives in PUNE. My mother stays with my grandmother and mentally retarded uncle and takecare of them. So my grandmother want to write a will of her property and her mentally retarded son property on my mother's name (second daughter who stays with them). Please advise the right way to proceed. Will be grateful for your help. Madan Mohan