Objective. To examine associations between family physical activity and sedentary environment and changes in body mass index (BMI) z-scores among 10-12-year-old children over three years. Method. Design. Longitudinal (three-year follow-up). Subjects. In total, 152 boys and 192 girls aged 10-12 years at baseline. Measurements. Measured height and weight at baseline and follow-up (weight status, BMI z-scores); aspects of the family physical activity and sedentary environment (parental and sibling modelling, reinforcement, social support, family-related barriers, rules/restrictions, home physical environment) measured with a questionnaire completed by parents at baseline. Results. At baseline, 29.6% of boys and 21.9% of girls were overweight or obese, and mean (standard deviation, SD) BMI z-scores were 0.44 (0.99) and 0.28 (0.89), respectively. There was a significant change in BMI z-score among girls (mean change=0.19, SD=0.55, p < 0.001), but not boys. Among boys, the number of items at home able to be used for sedentary behaviour (B=0.11, p=0.037) was associated with relatively greater increases in BMI z-score. Among girls, sibling engagement in physical activity at least three times/wk (B=-0.17, p=0.010) and the number of physical activity equipment items at home (B=-0.05, p=0.018) were associated with relatively greater decreases in BMI z-score. Conclusion. Sibling physical activity and environmental stimuli for sedentary behaviours and physical activity within the home may be important targets for prevention of weight gain during the transition from childhood to adolescence.