Purpose A pilot study to identify the predictors of improvement in emotional and behavioural functioning of siblings of children with special needs following participation in SibworkS, a six-week manual-based, cognitive-behavioural group programme. Method Data from 36 participants from a recent evaluation of the SibworkS programme was used. Measures were administered pre-intervention, immediately post-intervention and three months post-intervention. Treatment effects were measured using change scores for siblings on the Strengths and Difficulties Questionnaire – Parent Version (SDQ). Seven predictors were analysed: symptom severity of the child with special needs, participant age and gender, sibling birth order, family socio-economic status, participant baseline SDQ score and participant use of additional support services. Results The overall model significantly predicted change in SDQ scores at post-intervention and follow-up (adjusted R2 = 0.41 and 0.40). At both evaluation points, SDQ change scores were significantly predicted by baseline SDQ score. Furthermore, symptom severity of the child with special needs was a significant predictor at three months post-intervention. Conclusions Poorer emotional and behavioural functioning among participants and symptom severity of the child with special needs were associated with greater intervention effects. These results indicate that SibworkS is likely to be beneficial for siblings who have difficulties adjusting, and siblings of children with more severe special needs. Implications for Rehabilitation Siblings of children with disability are at increased risk of emotional and behavioural difficulties. Sibworks is a manualised group-based intervention for the siblings of children with a disability. Poorer emotional and behavioural functioning among participants and symptom severity of the child with special needs were associated with greater intervention effects. SibworkS is likely to be beneficial for siblings who have difficulties adjusting and siblings of children with more severe disabilities.