What is legal procedure to execute will, there are more than one will?

Sir, My father purchased property. After my father's death, my mother become the owner of the said property with the consent of all legal heirs. We are three brothers and two sisters and all are major. In Jan 2016, my mother execute a regd. will in favour of all the brothers and sisters. Thereafter, she again execute regd will in favour of sisters. After my mother's death, my sisters got property on the basis of will. Now my question is: 1. Whether the grand sons of my father can claim their share from the said property? If yes how? And in which court. 2. Whether certified copy of both the will is required for taking any legal action? If yes, can we take through RTI as we don't know the date of execution.