This study investigated family contextual effects on the association between screen time, behaviour and health-related quality of life (HRQOL) in child siblings. Data were from 1772 participants in the Australian Longitudinal Study of Women's Health 1973-78 cohort and their three youngest children (N = 4010 siblings) aged two to 12, collected in Mothers and their Children's Health cross-sectional sub-study (2016/17). The exposure was average daily recreational screen time (televisions, computers, tablets, mobile phones, electronic games). Outcomes were child overall behaviour and prosocial behaviour, and psychosocial and physical HRQOL. Multilevel models were used to test 1) the absolute effect of screen time, and 2) change in the child's outcomes when their own screen time changes relative to their siblings (within-family contextual effects) and change in the child's outcomes when the sibling average screen time changes (between-family contextual effects). The children were average 7.37 years (SD 2.76). There was an absolute effect of screen time on behaviour and HRQOL. Between-family effects were found for child total behaviour (0·22 (95%CI, 0.06, 0.37)) and prosocial behaviour (-0.10 (-0.17, -0.04)), but there were no within-family effects. For HRQOL, within-family effects were found for psychosocial (-0.97 (-1.91, -0.02)) and physical (-1.32 (-2.25, -0.39)) HRQOL, but no between-family effects were found. In conclusion, the higher the average screen time by all children in the family, the worse the child's behaviour, while the greater the deviation of the child's screen time from their siblings, the poorer their HRQOL. Family contextual factors should be considered in screen time research and screen time recommendations.