Placing property in someone's care without the transfer of ownership is a bailment. This has nothing to do with embezzlement.

OP, if your memory is accurate and you know that the valuables were in the locked box when you gave it to relatives, your recourse is to sue your relatives. How they got into the box and that they took the valuables is something you would have to prove. Is that something that you could prove or is it just your assumption?