Research on family factors in paediatric pain has primarily focused on parents; the role of siblings has been largely ignored. This study examined whether sibling relationship quality was related to siblings' behaviours during experimental pain, and whether the behaviours of an observing sibling were related to children's pain outcomes. Ninety-two sibling dyads between 8 and 12 years old completed both observational and questionnaire measures of sibling relationship quality. Children took turns completing the cold-pressor task (CPT) in a counterbalanced order with their sibling present. Pain outcomes (intensity, fear, and tolerance) were recorded for each sibling, and the behaviour of the observing and participating siblings during the CPT were coded as attending, nonattending, and coping/encouragement. Structural equation modelling, using the actor–partner interdependence model, was conducted to analyse the dyadic data. While participating in the CPT with their sibling present, greater levels of warmth and positivity in the sibling relationship were related to children engaging in more nonattending behaviours and less attending behaviours. Greater levels of attending behaviours by the observing child was related to the sibling having a lower pain tolerance, and greater levels of coping/encouragement behaviours by the observing child was related to the sibling reporting greater pain intensity and fear during the CPT. Children with warmer/positive sibling relationships were more likely to respond to acute pain by shifting the focus away from their pain experience (eg, through distraction) when a sibling was present. Pain-focused behaviours by an observing sibling are related to greater child pain and fear during experimental pain.