Objective: The aim of this study was to test whether the Intervention Nurses Start Infants Growing on Healthy Trajectories (INSIGHT) responsive parenting (RP) intervention, delivered to parents of firstborn children, is associated with the BMI of first- and second-born siblings during infancy.Methods: Participants included 117 firstborn infants enrolled in a randomized controlled trial and their second-born siblings enrolled in an observation-only ancillary study. The RP curriculum for firstborn children included guidance on feeding, sleep, interactive play, and emotion regulation. The control curriculum focused on safety. Anthropometrics were measured in both siblings at ages 3, 16, 28, and 52 weeks. Growth curve models for BMI by child age were fit.Results: Second-born children were delivered 2.5 (SD 0.9) years after firstborns. Firstborn and second-born children whose parents received the RP intervention with their first child had BMI that was 0.44 kg/m2 (95% CI: -0.82 to 0.06) and 0.36 kg/m2 (95% CI: -0.75 to 0.03) lower than controls, respectively. Linear and quadratic growth rates for BMI for firstborn and second-born cohorts were similar, but second-born children had a greater average BMI at 1 year of age (difference = -0.33 [95% CI: -0.52 to -0.15]).Conclusions: A RP educational intervention for obesity prevention delivered to parents of firstborns appears to spill over to second-born siblings.