Loan insurance paper copy

My father availed a loan from HDB bank but before we could get any documents from them my father passed away due to cardiac arrest. Later when we took the welcome kit from them we found that the insurance was took in my mother's name. My mother is a house wife and they made her sign the documents saying it is for the witness purpose and made her sign wherever my father signed. The loan sanction letter which we got from the bank was the only document which was with reference to both my parents and the someone else signed on it on behalf of my parents and return it to bank (fake sign). Now the bank officials are rejecting my father's insurance claim. I asked them to get me the copy of insurance application documents as I want to check if the sign on that too is fake or not. Because my father won't take an insurance in my mother's name as it won't serve the purpose.neither did my mother was aware that is a borrower.. Can I file a suite to get the copy of insurance application documents?because the bank manager is not doing it. And how to file for the same. Plz reply Thank you in advance