Children with 22q11.2 deletion syndrome (22q11DS), a copy-number variation (CNV) genetic disorder, demonstrate a great deal of variability in IQ scores and are at particular risk for cognitive difficulties, with up to 45% experiencing intellectual disability. This study explored the IQ relationship between individuals with 22q11DS, their parents and their siblings. Participants included individuals with 22q11DS, unaffected siblings and community controls, who participated in a longitudinal study of 22q11DS. Significant associations between proband and relative (parent, sibling) IQ scores were found. Results suggest that the cognitive functioning of first-degree relatives could be a useful marker of general genetic background and/or environmental effects, and can explain some of the large phenotypic variability in 22q11DS. These findings underscore the importance of including siblings and parents in studies of 22q11DS whenever possible.