First of all this is not ancestral property, but it was your grandfather's property.
Secondly, your grandmother has no rights to bequeath entire property in the manner she felt by making a testamentary disposition because upon your grandfather's intestate death, this property devolved equally on all his legal heirs consisting his wife and three children. Therefore your grandmother had only 1/4th share in the property, hence he cannot claim ownership to the entire property therefore the will is invalid and not enforceable as your grandmother did not have marketable title to the entire property left behind by your grandfather.
Next, during the lifetime of your mother, you cannot claim a share in it.
Now since the property devolves equally on all the residual three legal heirs of your grandfather, each heir shall be entitled to 1/3rd share in it.
If one among them is not agreeing to partition and divide the properties among themselves, then partition suit and relief of separate possession shall be the only option before you now.