Children who spend their early life in social-emotionally depriving institutions have limited opportunities to engage in relationships. This early experience has been associated with many problematic behavioral outcomes; however, researchers have not frequently examined relationship quality of post-institutional adoptees, nor have they examined aspects of the adoptive family that might moderate institutionalization effects. The purpose of this study was to examine the self-reported relationship quality of 10–17-year-old children adopted into the USA from Russian institutions and to determine whether sibling characteristics (relative age, gender composition, sibling adoption status) moderate institutionalization effects. Older age at adoption was related to poorer friendship and sibling, but not mother, relationship quality. Older siblings and same-sex siblings buffered children from this negative age-at-adoption association.