Ancentral house divided between two share holders can be sold to third party without consent of othe

We are having our ancentral living house of our grand father in Varanasi , U.P. After death our grand father it was mutually agreed between our self that ground floor area will belong to one party with open space around 35'×52' limiting his right up to 1st floor level ( 10') above to this height he cannot construct. 2- Second party is entitled for 2nd and 3rd floor area measuring roughly about 45'x52' each floor. 3- Now 1st party sold his premises to the 3rd party without taking consent from 2nd party. 4-Now questions arises whether it is legally right to sell his portion by 1st party then what action can 2nd party is entitled to take action against 3rd party. 5- 2nd party spoke to the 3rd party about his discontent in this matter. 3rd party assuring that once property will come under his possession he will sit with 2nd party and divide the entire property between two parts from bottom. 6- Now our interest what is legal option of the problem whether this purchase by 3rd party is legal or notice to be issued to 3rd party by 2nd property and what is the legal remedy of this matter. Thanks