Sale deed unregistered due to seller's daughter attorney.

I bought a land in 2013 and the sale deed was made which had everything ranging from stamp papers to signature of both the party's but could not be registered as the buyer ( X and his mother) could not produce the attorney of her daughter who had a undivided share in the property. This property was given to the son and his mother after the death of the father. They asked us for some time to get the attorney during which the cheques given by us as consideration were cashed by the seller. Agreed time passed but they din't produce the attorney. We were continuously asking them for attorney so that the sale deed could be registered. Soon the due time for getting it registered lapsed due to the fault of seller. Meanwhile the seller (including mother) left for Mumbai and died meanwhile. Now in 2016 the mother of seller who had also signed and fingerprinted in our unregistered sale deed came back and sold the property to the tenants residing in the property. She sold her part which was undivided. She also attached the map of the part of property which she sold. I want to ask is this thing fair? How can she do the sale deed again when she was at fault in our sale deed? She already cashed our cheques, so isn't this fraud? Someone told me that Supreme court through its ruling has made a decision regarding these cases of unregistered sale deed that the unregistered deed is deemed to be equivalent to sale deed and buyer is taken as the owner if money or the consideration is received by the seller and if due to his fault the sale deed could not be registered in past.