OBJECTIVE: We examined the direct and indirect effects of older siblings' substance use behaviors (i.e., cannabis and e-cigarettes) on younger siblings' later substance use intentions via their substance use expectations. MATERIAL AND METHODS: Data were collected from 682 families (N = 2,046) with two adolescent siblings (older siblings: M age = 15.67 years, 51% female; younger siblings: M age = 13.14 years, 48% female) and one parent (M age = 45.15 years; 85% female). Participants completed annual online surveys at two occasions. Older siblings reported on their cannabis and e-cigarette use frequencies (Time 1) and younger siblings reported on their substance use expectations (Time 1) and intentions (Time 1 and 2); parents reported on adolescents' sociodemographic characteristics and their own substance use (Time 2). RESULTS: Structural equation modeling results suggested that older siblings' cannabis and e-cigarette use was indirectly related to younger siblings' later intentions to use these substances through their positive expectations about substances, after accounting for younger siblings' earlier intentions to use substances and control variables including parents' and friends' use. There were no significant direct relations between older siblings' cannabis or e-cigarette use and younger siblings' intentions to use them. CONCLUSIONS: Findings indicate that older siblings are critical and unique socialization agents of younger siblings' expectations and intentions to use substances. Intervention and prevention programs that target adolescents' substance use should consider the ways in which siblings shape each other's substance use.
