Siblings represent an important social influence on alcohol use in adolescence. That said, there is a need for studies that examine potential mechanisms by which siblings exert an influence on the likelihood of drinking in adolescence. This paper illustrates a method that utilizes videotaped interaction between sibling dyads along with a micro social coding system that captures rule break behavior between siblings. Sibling interaction was observed in sibling pairs participating in the Iowa Youth and Families Project (IYFP) at baseline; younger sibling use of alcohol was tracked for 3 additional annual assessments. Exposure to older sibling rule break at baseline was associated with later use of alcohol by younger siblings across the 3 annual assessments. Micro social methods hold promise for uncovering processes that underlie sibling contagion for alcohol use in adolescence. [Additional early funding from the study came from the Bureau of Maternal and Child Health (MCJ-109572).]