Researchers from The University of Queensland and University of Washington have proved that tobacco and alcohol use by older siblings increases the odds of similar behaviour from younger siblings by three to five times.
University of Washington Sociologist Dr Abby Fagan studied the contributions and influence of parents, siblings and peers on teen drug use.
Dr Fagan used data from 1370 Brisbane teenagers, who've been part of one of the world's longest running health studies -- the Mater-University of Queensland Study of Pregnancy.
The teenagers were interviewed between 1995 and 1997 at 14 years old and were asked about how often they drank and smoked and also about their family relationships.
On average, 13 percent of younger siblings reported smoking and 36 percent reported drinking, but rates increased when older siblings also reported substance use.
About 10 percent of younger siblings with non-smoking older siblings used tobacco, compared to 40 percent of those whose older siblings smoked.
Likewise, younger sibling alcohol use increased from 25 to 53 percent when older siblings reported drinking.
"The results underscore the need to include siblings, or at least address issues relating to sibling relationships and influences, in prevention efforts," Dr Fagan wrote in her study, published in the latest American Journal of Drug Issues.
"Currently, most tobacco and alcohol prevention programs target individuals for change or are aimed at improving parent-child communication and interactions.
"If siblings are more powerful role models than parents, however, sibling and their potential influences on each other should be a primary focus of intervention."
Maternal depression also had a significant effect on adolescent substance use.