Sibling position is the idea that birth order predicts certain roles in a family. Roles the eldest, middle and youngest child will take on due to family expectations and different experiences.
It's cliché, but often parents are harder on their first child because they are new to parenting and want to get it 'right.'
By the time they have the 3rd child, the rules are more relaxed. As a result, the 1st and 3rd child may have very different experiences growing up and very different relationships with their parents.