An SSA Form 1699 must be signed by the Social Security wage-earner. It is generally used to appoint an attorney to represent a Social Security beneficiary. Social Security will recognize you as the proper person to deal with survivorship issues if you a court-appointed to be the Personal Representative of your bother's estate. Be aware that an appointment in a Last Will and Testament is just a nomination, a statement of preference for who should manage the estate. Nobody has the authority to deal with the estate unless that nomination is confirmed by a court of competent jurisdiction and Letters Testamentary are issued in favor of the executor appointed by the decedent. For the legal assistance you need, retain an experienced probate attorney.
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Unfortunately you cannot register online as your brother's appointed representative unless and until you have been actually appointed by a court. There may be other ways to get the minor survivorship benefits without having to be appointed by the court, as that is costly and time consuming.
I suggest you consult with an estate planning attorney with social security procedure knowledge. You will have to be appointed in court as executor to be his appointed representative. I would consult with a California attorney in the area to work thru this.