This study attempted to increase understanding of preschool children's reactions to sibling arrival. The aims were two-fold: First to assess how sibling arrival affected behaviour and internal representations of children's relationships, using a representational measure called the Storystem technique. Secondly, to explore the associations between children's storystem responses and behaviour, along with two maternal functioning factors implicated in affecting mother-child relationships and adjustment to siblinghood. 21 mother-child dyads participated in the pre-birth and post-birth home visits (mean age of children 60.14 months). Overall, two features of children's internal representations of relationships altered after sibling arrival: story resolution decreased and Disciplinary representations increased. Behaviour problems were unrelated to storystem responses pre-birth and did not increase across transition. Interestingly however, associations were found post-birth: more negative aspects of story responses were related to greater behaviour problems, while more positive aspects were related to fewer behaviour problems. In addition gender differences were found in children's storystem responses: boys' provided more negative stories than girls' and, across transition, became worse to suggest that boys' were more adversely affected by sibling arrival. No differences were found according to first or second time experience of sibling arrival. Maternal functioning was stable and high with respect to marital satisfaction across transition, while mothers' well-being improved. Storystem responses were largely unrelated to maternal functioning, except that greater marital satisfaction was associated with fewer Disciplinary representations pre-birth.