Land

In the late 90s A (my grandfather) purchased a land from B (his sibling) without writing any document regarding the purchase. B earlier purchase the land from C(some other villager) for which there'sa document. The money to purchase this land was lent by D (A's son-in-law) as debt. After many years E(A's younger son) and F (A's elder son) have repaid the debt along with A to D. After A's demise somehow F got the only document of that land which is on the name of B and forged B's signature and written a document stating F purchased the land directly from B. Recently E passed away and without E, his family is not able to get back the land as F is saying that E didn't pay a single paise. After several meetings with the village elders and others in the village E's family was told to bring D as witness. However F threatened to kill himself if D shows up to the next meeting. And D is scared of the whole blame. Now E's family is clueless. What can E's family do to get the land back which belongs to him.