Original Commentary
Brisbane and Sofia are five years apart...I get the feeling that Brisbane had to take care of his little sister a lot when they were kids, Terrence refuses to comment on this.

Modern Commentary
Brisbane always did get stuck as the caregiver, didn't he?
Given everything we know about him and Sofia, it fits. Sometimes, writing just means discovering things that were always there for the characters.
I want to say that Sofia is seven and Brisbane is twelve here. - Terrence