Laura. I do not think that Jacob's twin brother was the "MAN IN BLACK" nor was his adoptive mother. I think the MIB is what is inside the cave. If you noticed, the MIB did not appear in the episode until after Jacob's brother went under the rock inside the cave.
That means when the Smoke monster appeared, it expelled Jacob's un-named brother (who I'm calling OMEGA)'s body into the woods where Jacob eventually found him & placed him with his mentor's body in the other cave.
Now we know that Jacob would be visited by the MIB thoughout his lifetime & because his mentor told Jacob that Omega & Jacob could never kill one another, something else did in the cave & because it had taken on the form of Omega, it could never kill Jacob either.
Which is starting to relate why it took so long for MIB to kill Jacob.
Something else I got from the episode - Did anyone else notice that the guardians of the island are not just 2 entities, they are three (Good/Balance/Evil). This may explain why Jacob scratched out Kate's name on the rock, she, Sawyer & Jack are the three that MUST remain on the island!
Best Show Ever!
Lost is a very good story told very poorly. Sorry, but pacing and structure are elements of good story-telling, and Lost has neither.
There is more mystery than your recap. Was MIB born stillborn and Mother brought him to life with her touch, as Jacob is able to do? Did the Smoke Monster then inhabit MIB's body as a baby and then joint the Others to use them to get off the island as that was always his goal? Or was Mother the MIB? She single handedly killed all the Others? IF she was Smoke Monster, that would explain how she did that. So if she was smoke monster, then MIB wasn't smoke monster until Jacob threw him in the water. At that point smoke monster was able to inhabit his dead body, and the MIB we've seen this season has always really been smoke monster as MIB's body was buried in the cave with his mother. Lastly, Mother was stabbed and Dogon tried to get Smoke Monster stabbed when in human form as Locke so stabbing in the heart seems to be the only way to kill someone, and it has to be done before they talk, as MIB killed his mother. Good Show as it moved the mythology forward. This show is for learing not critiqueing.......................LOVE LOST