Please help with an honest suggestion regarding IPC 420, 406,120

Our family is in serious problem. Please help us with an honest suggestion regarding a fake IPC 420, 406, 120 B (I) case on my father. Before I convey my problem let me start from beginning. My grandfather (Scheduled caste) brought agriculture land in 1965 and after his demise this land was transferred (naamantaran) to his successors (5 brothers) including my father in 1985. Then they sold this agriculture land (SC) to registered society (non-SC members) with agreement duly signed by all heirs (all 5 brothers signed) in 1988. Then one of the heirs (uncle) passed away in year 1994 and during these years this land was not register in the name of society, so later on this land was again surrender to the society in 1998 with duly signed by all the 4 brothers and uncle’s 19 year old son (in place of his father). Recently, widow of uncle send a notice thru his lawyer to the society and my parents regarding surrender of land without her consent and signature. She demands the cancellation of sale agreement immediately and her share of land. She threatens to put criminal charges in IPC 420, 406, 120B (I) related to cheating & fraud acquisition against all the successors (4 brothers) except his son.