On the island, the likelihood of something happening has always seemed directly related to faith. Rose believed Bernard was alive. Eko believed he'd find his brother's plane. Hurley believed that 20+ year old Dharma van would start... and so it did, as ridiculously impossible as that scenario should've been. Even Locke's suicide note contained a very important phrase "I wish you had believed me."This basically goes back to Jacob's tapestry: May the gods grant you in all things your heart's desire. Do wishes come true on the island? Is that the secret "mechanism" operating on Lost?
So could the very act of believing in something make it true? And more specifically, could the firm belief in a set of rules actually bring those rules into existence? Jacob's brother even alludes to this, over a game of senet: "One day you can make up your own game, and then everyone will have to follow your rules." Is this what we've been seeing all these years? A game being played that's nothing more than a byproduct of Jacob's own design, with the MIB fighting for two thousand years to find a loophole in his rulebook? No wonder he's so pissed.
Another interesting thought, which also crossed my mind when watching the episode, was that Mother actually PLANNED to have the boys share the guardian duties. She herself, I think, was both Guardian and Monster. When she ended up with twins, she was in a way in trouble because as she told them, only one of them would get the job of guarding the light. So, in the end, when her "special" son betrayed her she ended up splitting the duties between the boys: Jacob became the protector, while the other son became the monster. I know we didn't see Mother as the monster in the episode, but too many things point to this being the case. If you guard the light, eventually you might just wonder what it's like inside the light and you'll end up becoming the monster. But Mother wanted to spare Jacob that, and he was spared. At the price of his brother.
As Vozzek puts it:
The ceremonial chanting and sharing of wine seemed largely symbolic to me. It was as if mother needed to convince Jacob that once he did this, his path was forever bound to the island. Jacob's still gullible at this point. He drinks up, and he believes her. Whether or not this truly does etch his destiny in stone remains to be seen, but this is where mother does something really, really slick: she recruits both Jacob and his brother to guard the island.
Jacob is 100% right. His brother was always first choice. But what he doesn't know: mother is shrewd enough to recognize that as 'good' as he is, Jacob can't guard the island alone. Jacob's honesty and commitment needs to be tempered by his brother's willingness to lie, be deceitful, and do anything needed to get the job done. Alone, each of them is only half a candidate. But together, they make an ideal guardian for the island's shores.