Property cheating

Hi Sir, This is regarding our partition deed in year 1983 (forefather's property). My father has got share 3.33 acre of agri land and as he was serving in defense so handed over to his elder brother to look after his land with no monetary benefits from him. His late elder brother cheated him and sold the 1 acre land to 3rd party in the year 1986 while he came on annual leave. This cheating and forgery was happened legally with my father's signature on Sale deed. My father didn't collect any money from 3rd party on this sale and all the money was grabbed by his late elder brother for his own need and expenses. My father did a big mistake of not getting into the insight of the sale deed and signed the copy without knowing the actual facts that his land was being sold. We actually came to know this fact recently in computerized encumbrance certificate. As his elder brother is not alive and only legal heir (his son) is available now. Please guide and share your comments if we can do any legal proceedings to get justice.