If you are looking for something you can do by hand, you can use a Vigenère cipher and memorize a fairly short key. This won't stop anyone who understands cryptography, but it should stop nosy family members. http://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher
You don't even have to do the enciphering by hand: http://sharkysoft.com/vigenere/ (Do inspect the code yourself to verify that the material being enciphered is processed on your computer and not sent to a server. I believe that to be true for the link given, but always look for yourself! Or, just do it by hand.)
Edited to add: Nosy brothers can probably crack simpler ciphers, such as the Caesar cipher.