Either your little brother registered already registered your phone, you mobile number was previously assigned to someone else, or someone accidentally typed in your phone umber when they registered for an account. In any case, the easiest thing to do to is:

AT&T, Cellular One, Sprint, and T-Mobile users text “reset pw” from your phone to 69937 (MYXER) and we’ll send your new password to your phone. (All other users please text “reset pw” from your phone to 9543046522)

Once you sign into your account, just go to the Account page and update the email address on the account.