Civil in nature

This is the problem. People sleep on their rights till alienated and then approach Lawyers. The moment Mother passed away, the brothers should have been advised by some knowledgeable relative/friends if any to get the property mutated in their favour and partition deed is duly got registered.  However, by the descripttion of the facts, the brothers must be very poor and may not be knowing anything. If atleast the Government order allotting the property to their mother is available now with the younger son, it should be examined whether the property is given with full rights or without the right of sale. That is the first step.  If it is proved that the Government order gives full rights, even then the sale is completely wrong and not valid.  

The buyer should have obtained proper documents while getting his sale registered. It is not known with what documents the registration had taken place. You may apply for public copy of the sale deed to see how the sale was executed and challenge it in Civil Court. There is no other way now.

File a declaration suit , making seller as party, for fraud and seek cancellation of Sale deed and seek partition of your share in the land. Local advocate can provide you further guidance.

The younger son's title to the property remains undisturbed. He can get possession of his share of his property and damages for the property destroyed.