The aim of the current study is to examine the quality of sibling relationships in relation to family functioning in Greek families with typically developing school-age children. The sample: 251 intact Greek families with two children (251 parents — 1 parent participated from each family — and 251 children). Research instruments: (a) the Family Adaptability and Cohesion Evaluation Scales III (FACES III; Olson, 1986), administered to both parents and children), (b) the Sibling Relationship Questionnaire (SRQ; Furman& Buhrmester, 1985); and (c) the family constellation: number of children, birth order, gender and socioeconomic level. According to the findings, the quality of a sibling relationship is associated with family cohesion and adaptability. Regarding children’s gender, the Warmth/Closeness scale is lower in families with children of different gender compared to families with children of the same gender. Regarding birth order, first-born children report higher levels of cohesion (ideal and actual), as well as higher Warmth/Closeness (children and parents), in comparison to the second-born children. Regarding parental educational level, parents and children report a higher level when it comes to ideal family and cohesion type (ideal level), as well as higher Warmth/Closeness (parents) when at least one of the parents has a university education background.