I'm the baby of three girls, and married a firstborn with one younger sister.

As a youngest, I tend to find the birth-order stories overrated. True, I was the slackest with homework, but it was the middle sister who was most diligent, not the older. We all graduated from uni; I got a PhD. My oldest sister was the party animal and she is perhaps a bit more bossy, but we're all very strong-willed and opinionated (in a good sense). Perhaps, it's less clear-cut in our case since I'm only 3 years younger than my older sister and the other is exactly halfway between us?

I'm also not convinced that the dynamic in our marriage is strongly influenced by birth order. Just my two cents.