FAMILY SUBSYSTEMS PREDICTING ADOLESCENTS’ PERCEPTIONS OF SIBLING RELATIONSHIP QUALITY OVER TIME Saralyn C. Ruff University of San Francisco Jared A. Durtschi Kansas State University Randal D. Day Brigham University This study examined associations between parents’ relationship conflict and parent–adoles- cent triangulation, and changes in adolescents’ perceptions of sibling affection and hostility. The goal was to learn whether conflict in parents’ relationships spills over to siblings’ rela- tionships, or whether siblings compensate by becoming less hostile and more affectionate. Using a subsample (N = 400) from the Flourishing Families Project (FFP), we found a trend for mother–adolescent triangulation predicting an increase in sibling hostility across 2 years. Fathers’ reports of relationship conflict were related to increased levels of initial sib- ling hostility, but predicted a marginal decrease in hostility over time. Findings support increased understanding of parenting dynamics associated with changes in sibling relation- ship quality, and have the potential to inform clinical practice. Sibling relationships serve as a child’s first social laboratory, offering endless opportunities for social interaction that shape cognitive, social, and affective development (Dunn, 2014; Dunn & Kendrick, 1982; Minuchin, 1974). They frequently have the longest duration of any relationship in one’s life, and serve unique functions that influence individual well-being. Moreover, interdepen- dent with other family subsystems, sibling relationships possess the capacity to affect, and be affected by, family processes and relationships (Brody, 1998). The aim of the present study was to examine patterns of interdependence between sibling rela- tionships, measured by sibling affection and hostility, and parents’ relationship conflict. Research indicates that parents’ relationships have unique influence in modeling dyadic affection and hostil- ity (Cummings & Cummings, 1988). In an effort to understand the nature and direction of these associations, this study evaluated whether parents’ conflict spilled over to increase sibling hostility and decrease affection, or whether siblings compensated for their parents’ relationship by becom- ing less hostile and more affectionate. We also examined the influence of parent–adolescent trian- gulation on sibling affection and hostility. Distinct from parental conflict, triangulation is a construct offered by family systems theory that refers to a parent’s attempts to form a coalition with a child that undermines or excludes the other parent (Bowen, 1976; Margolin, Gordis, & John, 2001). Our aim was to learn whether these two processes of parental conflict and parent– adolescent triangulation elicited similar or different patterns of interdependence, and if so, if these were in line with theories of compensation or spillover. Dr. Saralyn C. Ruff, Ph.D., is an assistant professor at the University of San Francisco in the Psychology Department and a licensed Marriage and Family Therapist; Dr. Jared A. Durtschi, Ph.D., is an assistant professor of marriage and family therapy in the School of Family Studies and Human Services at Kansas State University; Dr. Randal D. Day, Ph.D., was a professor emeritus in the School of Family Life at Brigham Young University, and the Principle Investigator for the Flourishing Families Project. Address correspondence to Saralyn C. Ruff, Department of Psychology, University of San Francisco, 2130 Fulton Street, San Francisco, California 94117, USA; E-mail: sruff2@usfca.edu JOURNAL OF MARITAL AND FAMILY THERAPY 1 Journal of Marital and Family Therapy doi: 10.1111/jmft.12265 © 2017 American Association for Marriage and Family Therapy