matter relating to relinquishment of deed

We are 5 children's for our parents. First is my elder brother, next is me, third and fourth are my sisters and fifth is my younger brother. I'm the first daughter. Both of my parents is no more. And The property was not distributed to anyone. So my father has died intestate. Now there is a land of 2 acres which is of 80 Guntas, which is undistributed. Coming to the core part our elder and younger brother made us to sign for relinquishment deed without consideration and promised to give only 8 guntas to each sister. So once the agricultural land is converted into sites, they would give us 8 guntas. Now the problem is we sisters want equal share in the property which is 16 guntas each. Can we revoke the relinquishment deed? If yes, on what basis? And if no what further steps to be taken for filing a suit on my elder brother? And most importantly younger brother is in favor of we sisters. The problem is from the elder brother. Kindly suggest . Thanking you in advance.