Agriculture land transferred by fraud

My Uncle aged 76 yrs, has 12 bighas land in his name.He has 2 sons and 1 daughter , all are married and live at different places.He with his wife live in his village and look after the land.Two of his 2 distant grandsons wrongfully , without paying a single penny got transferred his 3 bighas of land in the name their wives by 4 registered sale deeds , made in their names and signed by my uncle's wife as a witness. All the 4 sales were made in different days , 3 months back. His 2 sons had no information of this sale, when they came to know about they filed objections before mutation.Uncle was taken to the registrar 4 times and he signed all the papers in the front of the authority but without any payment of money in his name being transferred. Past consideration and past payments are shown in the registry. Uncle's sons went to police but it denied registration of FIR to his sons. Uncle's sons says that their father is blackmailed by the two of grandsons by having video clips of uncle. Uncle says he has not sold his property to anybody as don't need money.His sons have 3 sons , his real grandsons. Pl advice.