What responsibility does a bank have to provide a customer with suspicious activtity ?

My Mother had an account with her bank and my sister was the POA. My sister set up another account mom was not aware of and placed both their names on it. Alot of money was withdrawn from both. 8 and 9 thousand dollars at a time . This activitiy went on and on until my sister wiped out the accounts and left. I am now trying to understand why the bank and the fininancal advisor who sold her stock without moms consent let such activity go on and never informed My mother. My sister had done all the bill paying but was clearly abusing her authority. My mom is now broke and wondering if anyone can be liable other than my sister. I thought that elder accounts were watched more closely. When does a bank step in and call attention to insure that the main account holder is informed of the large amounts of money that is being moved. I thought a bank was supposed to protect your money and advise you of decisions that will affect you furture earning and saving. IS THE BANK AT ALL RESPONSIBLE