The two brothers seem to be representative of duality. The simple read is that Jacob is good and Brother is bad. But I'm starting to sense a more complex thing going on: The simple white and black represents how Jacob sees things, while MiB's outlook is more... complicated. Jacob represents dualism--the tendency to classify phenomena into "good" and "bad". Exemplified by the frantic impulse in monotheistic religions to categorize everything in the world either as holy/good/Godlike, and unholy/evil/Demonic. MiB represents non-dualism, the idea found more often in Eastern religions that everything in the world exists on an axis between the sacred and the profane, but all originating from the same source. MiB sees everything in shades of grey. He chooses to be with his Roman people, but he's not blind to the flaws in their character. He believes his "mother" to be hopelessly insane, but when she's right, she's right. Mother appreciates the mix of light and dark that burns so strong in MiB; it makes him special. It makes him the ideal candidate to stand astride this twilight realm between heaven and hell, taking on the souls of those who can't let go of this life, adding them to his strength to protect the source of all life. Jacob--poor, simple Jacob--played his part because accepted and continues to accept his mother's cover story. He sees himself as the good one and MiB as the bad one because he is incapable of seeing complexity. I guess he plays his part still in some way, as he endlessly scours the human race for "good" candidates. Meanwhile MiB rages helplessly, just fucking stop it. He's enraged at Jacob and enraged at himself for having unwittingly played into his mother's scheme.
There, I defy you to make sense out of that.