Will on Notary in the name of 2nd Sibling leaving others

My mother had availed services of a notary for writing a will during June 2016, We are 7 member Hindu family incl.5 children - 1 son (Eldest) & 4 daughters (younger). Father Retired Railway employee residing with 2nd daughter. 1st daughter, a divorsee, no offspring, stayed with mother since 1990. Eldest Son, married, living with family (Wife & son) in another state due to private employment. 2nd daughter, married, living nearby, husband private employment and 1 daughter. 3rd daughter, recently expired in an accident, having 2 children, husband self employed. 4th daughter, Govt Employee married to a Teacher, 2 children. Mother had entrusted all the property in the name of 2d sister (divorsee) on the grounds that she had looked after her since 1990. Rest of the children left out with nothing. Mother was expired during Sep 2016. None of the family members disclosed about will and not provided copy of will. Phone calls kept un attended. Messages left unanswered. What course of action, son can initiate at this juncture. Already tried to pacify father to advise others to share the property equally. None are ready to co operate.