This study examined how a firstborn child's empathy interacts with maternal emotional availability (EA) to predict later positive sibling relationships. The study included 108 families expecting a second child who also had a 10- to 45-month-old firstborn child (M = 24.6 months, SD = 7.42; 58 girls). Before the second-born child arrived, the firstborn child's empathic abilities were measured, and a mother-child play interaction was videotaped and coded for maternal EA. At 16-18 months postpartum, mothers completed questionnaires assessing the quality of the sibling relationship. Maternal EA moderated the link between the firstborn's cognitive and emotional empathy and the quality of the sibling relationship. Higher levels of emotional and cognitive empathy predicted better sibling relationships for children whose mothers were more emotionally available. Implications for early interventions are discussed. (PsycInfo Database Record (c) 2024 APA, all rights reserved).