“…Siblings are among the most important developmental influences one can have. Siblings impact each other's acquisition of interpersonal skills (Downey & Condron, ), cognitive development and sensitivity (Azmitia & Hesser, ; Prime, Pauker, Plamondon, Perlman, & Jenkins, ), emotional development and adjustment (Campione‐Barr, Greer, & Kruse, ; Kramer, ), social understanding (Dunn, ), sharing and prosocial skills (White, Ensor, Marks, Jacobs, & Hughes, ), socio‐cognitive reasoning skills (Slomkowski & Dunn, ), delinquent behavior (Bank, Patterson, & Reid, ; Criss & Shaw, ; Patterson, ), behavior problems in adolescence (Dunn, Slomkowski, Beardsall, & Rende, ), development of mental health and behavioral problems (Buist, Deković, & Prinzie, ; Tucker, Finkelhor, Turner, & Shattuck, ), and protection against the adverse effects of marital discord (Jenkins & Smith, ). Such is the pervasiveness of the sibling relationship, the strongest predictor of well‐being at age 65 among male Harvard alumni was found to be the quality of their sibling relationships during college (Vaillant & Vaillant, ).…”