Re partition and registered

My property is partitioned and Registered in 2002 under my grand father presence between A,B,C,D, -B-is mentally ill(unmarried) -D-is my father, even my father is not clever -B- is living with D since 30+ years. -B as some property , Both A and C combined together and Registered the Agreement in Sub register office, claiming for 80% of B property. -D as some property , Both A and C combined together and Registered the Agreement in Sub register office, claiming for 20% of D property, by blackmailing if you don't sign we will block the road -Now how can D get the property of B(because all this years D family is the Guardian of B) -And now B and D not willing to give any property to A and C, because they are cheating us by telling blocking of road and they are not paying single rupee, -And They are making us Scared telling agreement is Registered, so D can't transfer the property to me(son) -Are we need to cancel the Registered agreement paper?If yes, how long it will take in court to solve this case, and who's favor the case will go? --What can be done now please suggest, all the possible ways.Thanks in Advance