Siblings shape identity in ways that parents do not — as the first peers, the first rivals, the first mirrors outside the self, the people who know the same origin story and often remember it entirely differently. The positions within a sibling system — eldest, youngest, only, middle — create distinct psychological pressures and advantages that research consistently finds follow people well into adulthood. The sibling relationship is often the longest of a life, and examining what it created in you is examining something close to the bone.
Each step builds on the last.