• Want to challenge sales deed made by my father

There is a property (Begusari, Bihar )in the name of my grandmother. After my grandmother expired ,my grandfather executed a will in the favor of my four brother, which is registered will as per will act in the year 1994.
My father is adopted son of my grandmother & father. My father executed a sales deed in the year of 1996 for the said properties at that time we are minor and also my grandfather alive. My grandfather also become witness of sale deed and mention in the sale deed, I have read sale deed and I agree and done signature on sale deed. After my grandfather died in 201. We applied for probate in the year of 2006 in local court and order is passed by local court in 2009.
Now my questions are as below:
1) We four brothers have right to challenge the sale deed in the court; if yes please tell me the law and procedure for that.
2) can we sell any third party of said properties ,which is already sell by my father before probate order , what action should be taken for that and law applicable for that ?
3) Can also we can reverse other sale deed executed by my father before probate order?
4) Is there any necessary to my father also do sign to all sale deed which will be get executed by us in the future for buyer?