This study examined the degree to which adolescent self-system processes (self-efficacy, emotional reactivity) and reports of mothers' and fathers' behavior (warmth/support, hostility) predict adolescents' behavior toward siblings and their friends' problem behavior. Subjects were 76 seventh-grade adolescents who provided self-reports of parent behavior, self-system processes, and the criterion variables. Findings indicated that mothers' and fathers' warm/supportive and hostile behaviors were significant predictors of teens' behavior toward siblings. In addition, emotional reactivity and self-efficacy each made a unique contribution to the prediction of teens' hostile and warm/supportive behavior toward siblings. The combination of parent behavior and teens' self-system processes predicted 26-29 percent of the variance in teens' behavior toward siblings. The addition of parent behaviors to the model significantly enhanced the prediction of teens' behavior toward siblings beyond that afforded by self-system processes. This suggests that mediation of parent behavior through teens' self-system processes does not fully explain the pathways by which the quality of parent behavior influences similar qualities in teens' relationships with their siblings. Fathers' hostile behavior was the most salient parent behavior in the prediction of teens' reports of their friends problem behavior, and continued to make a unique contribution to this prediction when combined with teens' self-system processes. These findings are consistent with previous research that suggests that mother-teen and father-teen relationships may have unique effects on adolescents' development. (Author/KB)