You should call USAA or have your brother call. When I first joined USAA it was not open to former/current enlisted members however I was able to join through my father. When my children turned 18 the first thing I did was kick them off my car insurance and make them open thier own accounts with USAA. The oldest used my membership number to join and I actually got a call from USAA asking me to verify that she was indeed my daughter. The weird thing was the person I spoke to said they already had a record of her from being on my account since she turned 16. When my youngest daughter turned 18 last year she also got kicked to the curb( in the most loving way possible i assure you) but I was away on a trip when she went to sign up and wife was with me so she ended up using her sisters membership number as the current member for eligibility and was accepted and we never got ask to confirm but her sister got a email from USAA asking for confirmation. This may be due to the fact that they did indeed already have records that they were both my
children in the system. I do know that there is a section in your profile where you can add family members to your profile by there USAA membership number and siblings is an option to select but that may have nothing to do with eligibility.
Are you trying to get into USAA for the health insurance and all that? I signed up for a USAA account a while back and still have it but I am not military. I always thought that it was free to sign up as for certain things.
Full membership ( auto/home insurance ) requires a link to someone in your family tree or yourself who served in the Armed Forces at some point. While you can get most banking products from USAA as a limited member in the case of insurance you have to be a full member however some states require that Insurance Underwriters if they offer/provide insurance to anyone in the state they must then offer it to everyone, so you might be able to get insurance from them also depending on the state you live in. Its my understanding that they use a secondary insurarance provider for those policies and the rates are not as good.