Whethere is proof of tampering with locker or not, you can very well implead the Bank to be a party as the contents of the locker are the subject of a suit. This step should have been taken while filing suit itself.
It is true Lockers are not allowed to be operated by other than the person to whom it is allotted. If one of the claimants had obtained authorisation letter from the deceased while they were alive and locker is operated, nobody could be wiser.
Therefore, please consult Lawyer and implead the Bank issuing summons to Bank to produce the Bank Locker Operation dates and the copy of the signatures of the persons who operated with dates and times. This will clear doubts whether the locker is opened or not.