Challenge a registered will

My grandfather left a registered will in which he gave 65% property share to his elder son and 35% property share to younger son. My grandfather did not explain the reason in will that why he had given more share to his elder son. My grandfather's wife is alive along with 3 daughters. My grandfather was not aware of the contents of the will. If he was aware of the will he should have told his daughters that he has signed the will. But my grandfather did not tell his daughters or wife about the will because my grandfather himself was not aware of the will. My grandfather's elder son told my grandfather that the documents my grandfather is signing are not of will and the documents are of ration depot. So by fraud , my grandfather signed the will thinking that they are documents of ration depot. My grandfather's younger son used to take care of the my grandfather .My grandfather's elder son never took care of my grandfather , neither elder son did anything for my grandfather. My grandfather made the will in 2006 and my grandfather died in 2012. In this gap of 6 years , my grandfather never told his wife, his daughters and his younger son about the will because my grandfather himself was not aware of the will. If my grandfather was aware of the will, my grandfather must have told his daughters or wife that he has signed the will. And my grandfather made the will in the absence of beneficiary. The will is printed but the will has something written in handwriting. How can i challenge the will ?