A lot of research shows that birth order can impact the way a child grows up and who they develop into. However, others say it doesn’t determine anything. From what I’ve seen in my own family, as well as in close family friends, I think there is something to be said about birth order. It’s certainly not the be all end all - just like any personality theory, it can be taken with a grain of salt. 

What makes these relationships so important? When you’ve had a relationship with someone since birth, there is a level of authenticity that is formed. You have shared experiences, memories, etc. from your childhood.