Background A family systems perspective hypothesises that the well-being of all family members is interconnected. However, limited research has examined the association between inter-parental conflict and sibling relationship quality in the context of intellectual disabilities. The aim of this study was to build on existing literature using longitudinal data, exploring potential (bi)directional associations between sibling relationship quality and inter-parental relationship satisfaction in families of children with intellectual disabilities. Methods and procedures Primary caregivers (n = 223) of children with intellectual disabilities (M age at Wave 1 = 8.36 years, SD = 2.33) with at least one sibling (M age at Wave 1 = 8.45 years, SD = 2.47) completed the same online survey, 2 years 9 months apart. The survey measured inter-parental relationship quality and sibling relationship quality. A cross-lagged panel design was used to determine the directional associations between both subsystems. Outcomes and results The final autoregressive and cross-lagged models had good model fit. However, we found no evidence of any directional associations between sibling relationship quality and inter-parental relationship satisfaction. Conclusions and implications The results from the current study contrast with previous literature, exhibiting the need for further research exploring factors associated with subsystem relationships in families of children with intellectual disabilities.