How to settle the property division

Hi there, My grandfather has a property in Delhi. He died in 1987 and left a will notorised executed at Jaipur. My Grandfather had 6 children, 3 sons (2 have expired in late 90s) and one alive son.3 daughters all present currently. The property in question consists of 2 floors, where two of his sons family lives. (one Deceased and one alive son).Both of these parties approached rest of the people to settle the property amicably as per Grandfathers will which stated that this property should be settled in 1/3 proportion each to his three sons. We requested the 3 aunts to sign a Relienqishment deed favoring 3 brothers which they are not signing ( and ofcourse consider themselves 1/6 owner in this). What the 2 brothers family's who are currently residing there to demolish the said property and rebuild as the construction is quite old and offer the third brother family also to construct the third floor and enjoy the peaceful possesion. But the problem is that even if we invite the third brother family to come and stay and all three spend money on this, it still wont be legally ours and how do we settle this. what remedy is available to us legally.