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You would need copies of his birth certificate and your birth certificate to prove to the Mexican authorities you are his brother and next of kin, which means you would have to declare he is not married and has no children as well. They require proof of your relationship and the US Embassy there will provide you assistance with any translations required as well. See also: http://mexico.usembassy.gov/eng/citizen_services/death-of-a-citizen.html
I'm his sister. I have his birth cert. Mine and my marriage certificate. It shows we have the same parents. Is that enough for to also gain access to his storage locker ??