Yes, it would be a violation of the law IF your brother knew his POA had been revoked: if he knew that, then he intentionally or knowingly took property to which he is not entitled, which is theft.If he did not know of the revocation, then he did not commit the crime of theft, since he would have lacked the requisite criminal state of mind or intent ("mens rea"); however, your mother could stll due him for the return of the contents of the box or their value, if he does not voluntarily return the contents.