You have not stated that how did your father acquire the landed property.
Assuming it to be your father's self acquired or inherited property, it can be termed that it was your father's own and absolute property.
You say that you and your brother have been staying out of the town for 15 years or more, so you weren't interested in the property till it was sold to a third party.
You must become aware that since it was your father's own property he had full rights to sell the same in the manner he desired and to the person of his choice.
Your father's authority to sell his own property cannot be challenged even being his son.
If you suspect a foul play and have evidence for that then you can ask your father to file a suit to cancel the sale deed, you cannot directly involve in this litigation during the lifetime of your father.
You may consult a local advocate on all such further issues.