My best friend when growing up came from a family of 12, spanning nearly two decades in their ages. Older caring for younger was clearly part of the environment in their household. Yet there were distinct differences in personalities and interests.I have read that identical twins can be quite different in personalities but there is also some kind of unseen connection between them so, IMO genetics and environment seem to both influence human development.
When I was young, I lived next door to a family of 9 children. I used to go over there all the time because there was always a lot of diverse things going on as compared to my family with just myself and older sister. I observed the older siblings caring for the younger ones and the parents not having to do as much for the younger children. Their main 'job' was to make sure the older siblings passed along family edicts and values and did not abuse the younger ones.
When growing up in a small town, my mother's two sisters and their families, along with an aunt closer to her age, all lived within a couple of blocks of each other. Tally of me and my siblings, along with the cousins, and there were close to two dozen of us growing up together. Ages ranged a bit over a decade worth of span. Wide range of personality differences.