Sibling effects refer to the immediate influence one sibling may have on another or to indirect influences through their embeddedness in a common friendship network We used three aspects of sibling mutual interaction—warmth, conflict, and frequency of contact with mutual friends—to evaluate sibling effects on delinquency and substance use in 135 brother pairs, 142 sister pairs, and 141 mixed-sex pairs in the Arizona Sibling Study (primarily aged 10–16 years). We hypothesized that sibling relationship variables would condition the behavioral resemblance of the younger and older sibling. For both substance use and delinquency, this prediction was confirmed for warmth and mutual friends: Sibling pairs who reported warmer mutual relationships or greater contact with mutual friends were more alike behaviorally. The statistical sibling effects were not explained by social class, parental substance use, or rearing styles. We interpret them as the influence of one sibling on the other and as the influence arising from sharing common friends. Given the existence of sibling effects, the strength of shared familial influences of other origins must be revised downward.