Ancestral property sold illegally

My great grandfather had land of 22 acres, he died without transferring the land to his two sons(my grand fathers), later my elder grand father died for some ailments, and my younger grand father who brought up his only brother's son(who is my father) from his childhood. When my father at his 18-20 age, my younger grand father sold land to someone by blackmailing my father as he would die drinking poison if not signed on paper. My father feared and signed wherever my younger grand father asked to sign(30+ years back). Recently I was checking the documents of other land of my family, and got to know that there is no papers about this land(22 acres), when checked in registrar office there is two registers on the same land written by different document writers(both shows same details), shows it was sold to someone by my grandfather and father, and there is no document which show the land was transferred to my father and grand father from their fathers. Now my question is how can my grandfather can sell land to some one when the land was not transferred to his name or in joint with my father, after my great grandfather and grandfather death. Is there any way that we could get out land back. Great grandfather(Died) | _______________________________________________________ | | | Elder grand father(Died) Younger grand father(70+ now) 2 daughters(1died,70+) | | Only Son(My father) (50+now) _________________________ | | | 2 Sons(me and my brother) 4 daughters(1died,others30-40+) 2 Sons(30-40+)