Can a bank be sued for letting my dad's wife into his lock box with just a key and she was not on the signature card?

My dad passed away last year from pancreatic cancer. About 2 weeks before his death, his wife of 8 years took dad's key and got in his lock box and emptied it. She had never been on the signature card before, but she persuaded the bank manager to let her in. He agreed to let her in, but she would need to bring back a lease agreement with my dad's signature. So he let her in right then, and she took everything. My dad had stock certificates, their pre-nuptial agreement, his latest will, cash, his golf championship ring and I don't know what else. His wife saw that dad had removed her as executrix when he redid his will naming my brother as executor. Dad's wife says that dad tore up the new will and wanted to use the will where she was named executrix. This supposedly happened 2 weeks before his death. He was on very strong narcotics including fentanyl, morphine, and delaudid. He was not aware of anything going on. Dad's wife took his 2 life insurance policies and did a change of beneficiary on both policies naming herself as the beneficiary. She forged my dad's name, and luckily we notified the insurance companies as to what she had done. My brother and I did receive the money. We also got a handwriting expert to determine if it was dad's signature or not. She said it definitely was not his! We have an attorney, and the bank manager and dad's wife have been deposed. They both lied the entire time. Eventually the banker did admit to letting her in without my dad or my brother’s knowledge. My brother was my dad's POA. This woman is master manipulator. She is a narcissistic sociopath! By the way, my dad is her 4th husband to die. Here's my question: can we not sue the bank for what they allowed her to do? What they did has totally ruined our life! We have nothing of our dad's because she got rid of everything. She is the one who tore up the will, but we don't have a copy to prove what my dad had told us.