My question 1 : Can we ( me and my mother ) question the sale of sites by my father without our consent.
The property that belonged to your grandfather was duly partitioned by him among his sons, thereby his sons have taken possession of their respective shares. Thus they become the absolute owners of their share in the property.
This being the case, nobody can claim any share in their share of property as a right during their lifetime.
Therefore neither you nor your mother do not have any rights in his property and hence your father is not required to take your consent to sell his own properties.
My question 2 : Is my father eligible to sale the site on 2006 when my grand father died in 2004 ? I mean, GPA becomes invalid after my grand father death right ?.
No doubt the GPA is invalid after the death of the principal, whereas who will contest this, the legal heirs who are entitled to a share in such properties only can contest it and not by a third party who has no interest in it.
If your father sold the property in the capacity of the GPA after the death of the principal, then the legal heirs of the principal i.e., your father and his siblings only can raise an objection to this.
Since your father is one among the legal heirs and the other heirs also have been allotted with their respective shares in the properties, there can be no claim towards this from their sides.
In any case you dont have any rights in the properties hence you cannot raise any objection to this.