challenging a property will

Can i challenge my grandfathers will? my grandfather has 3 son (including my father) and 3 sisters. my grandfather died in 2010. my father is eldest son. he was living with my younger uncle. he didn't tell any of his child of any will. we waited four years. afters four years of death of my grandfather we (my father & other uncle) went to talk with younger uncle, he showed us grandfathers will which were totally unknown. we feel that it was made under influence. the sign is not staight. there are sign of two witnesses. now can we challenge the will. my aunties are not willing.