So it’s pretty well established that birth order does make a difference in personality; the ‘firstborn’ will be different than the middle children, who are different from the youngest. I’ve read that children who are more than 5 years younger than their next oldest sibling exhibit the qualities of a ‘firstborn’ as well, meaning that with a 5 year separation, new children are effectively a new set of children, not significantly influenced by their older siblings.
All of this means that if my boys are going to have any more genuine siblings, their new sibling must be conceived in the next year.
Then again, I’m sure it doesn’t matter. ;]