Ancestral Property with common areas to 2sons.1son died, can half be sold to third party?.

My grandfathers property( built by him &his two sons) was given to his sons after 15 years of his death. The prop is 120 yard land with 3 stories, the front 3 flats given to elder son(my uncle) & behind 3 flats given to my father. With one hmws connection, boring pump & terrace & verendas in common for both. While sorting the property to the two sons, the elder one(my uncle) said acc to my grandpa wish we must stay behind half which is not so pleasing to the tenants considering darkness etc etc. when there was a society meeting,as we needed money and a common prop will not fetch loan from banks, we proposed alternate floor ownership to which they disagreed and finally both sons agreed on selling the prop and taking the halfs(have this written in piece if paper signed by them,us &members if meeting). Later they denied their interest in selling their half and lied they never accepted in selling as it was our need. Now, MY FATHER DIED A WEEK AGO. We are tensed as we are in debts all kver and have our sister. If we sell our half prop to him,We heard he wanted to pay 30 lacs for a 90lac worth place. They cornered us.Can we sell our half to third party for a better price? How do we solve this so we can get a better take... the mental torture of debts, these ppl is killing us. We would prefer selling it for better price and saving for our sisters marriage and buying a new home. How should we proceed?