Settlement of the father property to younger son second marriage.

My grand father had 3 sons and the whole property was divided in 4 parts t.e for 3 sons and my grandfather. During the division of the land property the oral discussion amongst the four was that the father property will be distributed equally between the 3 sons. But in due course the youngest son wife was expired and my grandfather have gone for the second marriage of the his youngest son and written a settlement deed of 1/3 rd of the grandfathers distributed property on the stamp paper without the concern of the other two sons. And the settlement deed was not registered even today. the whole property was the contribution of all the 3 sons and their father as a joint family. My grandfather bears the pattadar pass books on his name even today. My question is that is that settlement deed is valid without registration. and also all the three sons and their heirs signatures are not there on the settlement deed. What is the life time of the settlement deed without registration. Can I approach the court being a grand son for justice