Okay, I'm completely baffled now. I though I had it figured until Jay/Jeremy mentioned seeing two boys. I was convinced that Oliver was actually the made up brother. David was the son of Brian and Mary, he was rebellious and difficult and caused them many problems. During some sort of fight with the parents David was seriously injured and had been hospitalized. When he recovered however, he had created the Oliver personality. Brian and Mary like Oliver and have been trying to keep him over David as their son.
Oh well, I guess that is why this story is so great, you just never know. I'm working on a new theory, but need to read ch. 11 to see if it's heading the same way.