Abstract Background Being a last-born child and having a sister have been associated with higher body mass index (BMI). Encouragement to eat that overrides children’s self-regulation has been reported to increase the risk of obesogenic eating behaviors. This study sought to test the hypothesis that encouragement to eat during mealtime from older siblings and sisters mediates associations of being last-born or having a sister with higher BMI. Methods Children aged 4–8 years ( n  = 75) were videotaped while eating a routine evening meal at home with one sibling present. Encouragement to eat (defined as direct prompts to eat or general positive statements about food) delivered to the index child (IC) from the sibling was coded from the videotape. Path analysis was used to examine associations between IC’s birth order, sibling’s sex, encouragement counts, and IC’s measured BMI z-score (BMIz). Results Being the younger sibling in the sibling dyad was associated with the IC receiving more encouragements to eat from the sibling (β: 0.93, 95 % confidence interval (CI): 0.59, 1.26, p