What are the rights of daughters on father's property?

Dear sir, My grand father died 2 years before,before that my grandmother died,few days before my grandmother's death my father's sisters without consulting my father his 3 sister make a will and taken a signature of my grandmother on that will and even my father's signature also not in that will ,so today they have sended a legal notice that they have share in that property so suggest some legal action which we should be taken and what proofs needed to be carried out from our side