LITTLE BROTHER, WHERE DO I BEGIN?

O, little brother, where do I begin?
Its a story, an allegory, of unoriginal sin
There wasn't room for another within the womb of our mother
so I absorbed you into me, my little twin brother
Not quite alive, yet not fully dead
You're a rumor of a tumor living inside my head
I hear you scream out curses of all kinds all the time
shrieking verses in my mind at the tormenting divine
within your prison, your forsaken and unholy shrine
yet you've risen to take over half of my mind
A material ethereal presence, refusing to bend
A nemesis of symbiotic essence, my evil twin
So tell me, dear friend, where does your consciousness end
and where, little brother, do I begin?