Hm, I don't know. If it helps the theory at all, my older sister (half sister) never lived with me or my two younger siblings, essentially making her fill the role of only child and me of eldest, in terms of environment.
I don't think that having to be responsible makes one a J. I think people have an innate preference. I'm the oldest, and a hardcore P. Like, my parents and sister are convinced that I'm thisclose to being homeless, b/c real life stuff doesn't hit my radar that often.
Accept the past. Live for the present. Look forward to the future. Robot Fusion
"As our island of knowledge grows, so does the shore of our ignorance." John Wheeler
"[A] scientist looking at nonscientific problems is just as dumb as the next guy." Richard Feynman
"[P]etabytes of  data is not the same thing as understanding emergent mechanisms and structures." Jim Crutchfield