Primitively social Sclerodermus parasitoids form female multi-foundress groups on large, paralysed hosts and then cooperatively care for large broods of offspring throughout their development. We identified offspring with microsatellite markers and found that nearly every foundress produced offspring, implying that overall reproductive skew was thus not extreme. We expected that kinship may influence skew if, for instance, relatives within a group collectively suppress the reproduction of a non-relative or if non-relatives selfishly over-exploit group reproductive optima. We found slight skew, in favour of the non-sibling, when two full-sibling foundresses brooded with one non-sibling foundress but no skew when three sibs brooded with one non-sibling. All foundresses made similar contributions to the sexual composition of broods, which were highly female biased. We expected body size differences might promote skew via physical domination of some foundresses by others or via intrinsically differential fecundity. We found heterogeneity in the relative contributions of dissimilar-sized foundresses but no skew overall. Nonetheless, males within these broods were more often the progeny of the largest foundress, suggesting that larger females are able to dominate the production, or the survival, of males. We expected that temporal priority may generate skew via early individuals having a longer period in which to reproduce and/or become dominant. We found that the skew indeed favoured foundresses that had started to reproduce before a second foundress was introduced. Further, if eggs laid by a first foundress had started to hatch before a second foundress was introduced, the second foundress produced female offspring only. Overall, we found that skew is not strongly associated with kinship among foundresses, but does appear to be associated with intrinsic or acquired dominance and that where skew is manifest it can comprise brood sex ratio effects as well as overall reproduction effects.