Sibling personality is another world from only child personality.
I think, siblings simply have more impact than we might think.
After-all, an only child spends 100% of their time at home watching and learning from adults.
But with a few siblings in the picture, only a fraction of what is heard, seen, and learned is coming from an adult or parent.
Niche picking might help to make sense of this mystery. This concept has its roots in the work of the physician and psychotherapist Alfred Adler, who came to believe that Sigmund Freud’s emphasis on parenting failed to account for the critical influence of siblings on personality development. Adler argued that because firstborn children start life as only children, they initially identify with their parents. When a younger sibling arrives, firstborns risk being “dethroned” and often respond by emulating their parents: they enforce rules and assert their authority over the younger sibling, which sets the stage for the younger child to rebel.
-Adam Grant, Originals