This study identified behaviors of sibling pairs interacting on a cognitive task and related these behaviors to sibling structure variables (age and sex of each sibling and age spacing between them) and to measure of cognitive abilities of the younger sibling. Subjects were 160 sibling pairs randomly selected from appropriate subpopulations of two-child families: half the younger siblings were kindergartners, half second graders; half the older siblings were two years older, half four years older. The four possible sex combinations were equally represented. For half the pairs, the younger sibling carried out an initial object-sorting task alone. For the remainder, the older sibling helped the younger on sorting, and their behavior was recorded. Measures of the younger siblings' categorization abilities on a second object-sorting task were taken. Results showed that the assistance of the older sibling had a significant effect on the younger sibling's later categorization for six of the eight object-sorting variables. Children with siblings four years older made larger groups and left fewer ungrouped items than children with siblings two years older. Children with siblings of the same sex used fewer relational categories (according to the Wallach and Kogan criteria) than children with opposite sex siblings. Significant correlations were found between siblings' behaviors and the younger siblings' later categorization. (Author/KM)