psychotherapy:

“Your parents leave you too soon and your kids and spouse come along late, but your siblings know you when you are in your most inchoate form. Assuming you all reach a ripe old age, they’ll be with you until the very end, and for that reason, there is an intimacy and a familiarity that can’t possibly be available to you in any other relationship throughout your life. Certainly, people can get along without siblings. Single children do, and there are people who have irreparably estranged relationships with their siblings who live full and satisfying lives, but to have siblings and not make the most of that resource is squandering one of the greatest interpersonal resources you’ll ever have.”

– Jeffrey Kluger, The Sibling Effect