Operated deceaseds' account via ATM post death, legal repercussions?

Operated Deceaseds' Account Via ATM Post Death MY FATHER EXPIRED ON 1/09/2017 DUE TO DEFICIENCY OF SERVICE BY BANK THE BANK CONVERTED JOINT PENSION ACCOUNT INTO PERSONNEL ACCOUNT AND PERSONNEL ACCOUNT INTO PENSION ACCOUNT I AM NOMINEE OF PERSONAL ACCOUNT OF MY LATE FATHER. THE ATM CARD WAS IN POSSESSION OF MY MOTHER AND ON MY MOTHERS DIRECTION AS GUIDED BY FAMILY MEMBERS MONEY WAS WITHDRAWN AFTER DEATH OF MY FATHER BY MOTHER TO PERFORM POST RELIGIOUS DEATH FUNCTION TO GET THE BODY RELEASED BY HOSPITAL. NOW THE BANK SAYS THAT THEY WILL FILE CHEATING AND THEFT CASE AGAINST US. CAN THEY DO SO ,CAN THEY FILE FIR DIRECTLY IF I LEGAL NOMINEE DOES NOT HAVE ANY OBJECTION OF MY MOTHER WITHDRAWING MONEY WHAT IS THE REMEDY AND SOLUTION AND OPTION AVAILABLE BEFORE ME THE BANK ARE INSISTING THAT I SHOULD GIVE IN WRITING THAT MONEY WITHDRAWN BY MY FAMILY MEMBER. IS THERE ANY HARM IF I GIVE THIS IN WRITING