Well, we finally know that the-one-whose-name-cannot-be-pronounced and Jacob are brothers. That was the most obvious aexplanation in the beginning, when we first saw the monster in the form of Locke and realized what were his intentions towards Jacob. That is, this was maybe the first theory people came up with to explain what we knew at the time. If I remember, there was something like "only brothers, or at least relatives, could be so angry at each other at the point of one wanting to kill the other, but, even so, being unable to do it by their own hands". It is interesting how the writers deal with an obvious thing to make it so un-obvious to us! So many theories, so many dispute, arguments... and yet the most simple and human explanation is the real - and the best - one. Even if it is a subject already so exploited in other works - remember Star Wars and Supernatural, for instance - they have been able to do it in such an original way that it definitely does not as an old, wore and exahausted theme.