Partner selling disputed property without concent

My father did a timed agreement with the party , on our land the party could do the construction and after the sale of the flats constructed the profit will be divided accordingly. My father died soon after the agreement and the party stopped the construction. Having bad intentions of taking over the whole plot on low cost , the party did a case over us mentioning to return their investment with interest which they knew we would not be able to return and hence forcing us to sell them the plot at low cost in return for taking the case back. Now the court order stated that we have to return their amount with interest. the only way to return their amount for us was to sell the land with the party and repay them.this remained the situation for few years as there was no potential buyer. Now the situation is , we were 2 bothers and 1 sister who were owners of the land after dad died. The party purchased 1/3 of the land from one of our brothers on very low cost by illegally transferring the land on the 3 of our names as the other 2 of us were not present while the transfer on our names happened.there is no demarcation on the land mentioning which part belong to who. Now the party is trying to sell his 1/3 part of the land without clear demarcation , without our concent . What can we do in this case?