The property that belonged to his father shall devolve equally among all the legal heirs of the deceased if the deceased died intestate.
Under the above circumstances, the sisters of land owners have equal rights in their father's property though the land owner has transferred all the records in his name by bribing the concerned authorities.
The land owner can sell only his undivided portion of share in the property and he cannot sell the entire property, therefore the sale deed insofar as his sisters' share is concerned, is illegal and invalid in law, deserving to be declared as null and void.
The sisters' subsequently cannot execute a GPA in favor of anyone for the entire property, they can restrict their claim upto their share alone.
If the situation appears for a peaceful settlement, negotiate the issues and go for a compromise or file a suit for declaration and injunction owing to the threats and pressure by some third persons after which they will come out with their claim upon which you can ask for refund of your money with interest.