Truthfully, there are probably huge differences in the operant influences that you and your brother have experienced. Granted they are probably more similar than the way two strangers would have been raised, but this is merely a question of degree...
You may have been "raised" in a way that seems similar, but in which an objective observer would probably find subtle but important differences. In addition to what Mashy brings up, you and your brother probably have a set of people with whom you both have regular one-on-one interactions (family and such), but these interactions (to which the other is not directly privy) are not identical. You have (or have had) different sets of friends, different cohorts in school, etc., etc. Also, do not discount the fact that two people may percieve the same event/conversation in different ways and thus be shaped differently by the same experience.
Don't get me wrong, I am a big proponent of the idea that both "nature" and "nurture" factor into personality development. I just don't think you're giving "nurture" enough credit
SS
Alric has bats in the attic. ~Larisa