Fraud And Blackmailing To Evade The Friendly Loan

A relative of mine intentionally took a friendly loan from my mother bank account into his wife bank account through bank transcation an amount of 5 lakhs, and again 6 lakhs in small amounts. he made an invalid promissory note intentionally as he knows he is going to cheat at the end, he dint pay atleast one month interest for the past 4 years, we are unable to send any leagal notice because they are frequently changing their location as they are in rented house. Recently they bought a own house of 1 crore rupees and car worth 10lakhs and now they completely denied taking the loan as the promissory notes are invalid but we have the bank transactions, when we demanded money or we will file case they agreed that they will return the money soon and now they had blocked our phone numbers. we have the proofs like, call recordings in which both the wife and husband agreed they took money and they will return, bank transactions receipts, invalid promissory notes. Can we file a criminal case under 420 as they intentionally cheated. in the arguments over phone once he intentionally verbally abused my mother and provoked me then i too verbally abused his wife and daughter on phone and he recorded these words and now blackmailing me that if i ask money again he will file a criminal case against me on dafamation grounds. Please explain all the possible solutions, we need them to put behind bars in criminal and blackmailing case at any cost or it is better to forget about the money because i have used the foul language in the argument. is it a crime to use foul language when someone provoked by insulting ones mother.