We put enormous weight on how parents shape a child and almost none on siblings, but the research suggests brothers and sisters do quiet developmental work of their own, including buffering each other through the hardest stretches of childhood

Almost all of the attention paid to a child’s development goes to the parents. Parenting styles, attachment, the thousand small decisions of raising a person. Siblings tend to get treated as background noise, the people who happen to be in the house. Yet for …