While offspring size is a widely studied concept in evolutionary ecology, mechanisms affecting offspring phenotype in species with post-zygotic parental care are incompletely understood. We examined the impact of sibling fusion on ontogenetic shifts in offspring size in the brooding sea anemone Urticina felina. Fusion occurred among brood-protected embryos in U. felina, whereas it occurred post release among settlers of corals studied here and previously. Two fusion products were evidenced: morphologically-aberrant offspring and large homogeneous offspring coined ‘mega-larvae’. The frequent occurrence (~77%) of mega-larvae identifies them as the primary fusion product, which drove an increase in offspring size and within-clutch size variation prior to release. Lipid signatures suggest that morphologically-aberrant juveniles represent by-products that do not reach adulthood. Not only were occurrences of mega-larvae common in the populations studied, they increased with maternal fecundity, suggesting that sibling fusion may be a form of kin cooperation integral to the reproductive success of U. felina, warranting investigation in other live-bearing invertebrate taxa.