Joint name bank locker dispute - Mother/ Daughter

My mother and sister have a bank locker in joint name where they have kept their jewellery. My sister only used to operate this locker. My sister expired and the bank locker keys are with her husband. We have requested him many times to come to bank with us and resolve this however he is always ignoring us. My mother has worked very hard to make little jewellry for herself and now she is completely devastated by my sister's death. As per bank, keys and heir of deceased is required to close the account. Kindly suggest what can be done.