While offspring size is a widely studied concept in evolutionary ecology, mechanisms affecting offspring phenotype in species with postzygotic 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 postrelease among settlers of corals studied here and previously. Two fusion products were evidenced: morphologically aberrant offspring and large homogeneous offspring coined "megalarvae." The frequent occurrence (∼77%) of megalarvae identifies them as the primary fusion product, which drove an increase in offspring size and within-clutch size variation before release. Lipid signatures suggest that morphologically aberrant juveniles represent by-products that do not reach adulthood. Not only were occurrences of megalarvae 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.