Oldest siblings are the most persuasive models for or against delinquency for adolescents.
In a study of Latina teens, if a girl's mother had been a teen mother, she was only .2 times more likely to be a teen mother herself. However, if a girl's older sister had been a teen mother, she was 4.8 times as more likely to become a teen mother herself.
Drinking, smoking, drug use, and crime are almost as contagious from older to younger siblings. The effect is stronger among sisters than among brothers.
These findings are reported in Jeffrey Kluger's The Sibling Effect.
This suggests to me that interventions and ministries with at-risk youth should focus on the eldest children in a family most of all.