Parents and siblings play a vital role in promoting and inhibiting the development of comforting behaviour -- a typical prosocial behaviour. This study aims to explore the dual mediating model of positive and negative sibling relationships between parental self-efficacy (PSE) and firstborn children's comforting behaviours. We enrolled 163 firstborn children aged 4-6 years. The firstborn children scored significantly higher in comforting behaviour in a 'toy damage' task than that in a 'physical harm' task. Moreover, PSE promoted the development of children's comforting behaviour through a positive sibling relationship and weakened the development of children's comforting behaviour through a negative sibling relationship, revealing the internal mechanism of sibling relationship's two-way influence on firstborn children's comforting behaviour. Comparatively, no significant difference was noted in the mediating effect between positive and negative sibling relationships, providing clues for future intervention studies.