Cross-lagged panel designs were used to examine longitudinal and potential (bi)directional relationships between primary caregiver reported sibling relationship quality and the behaviors of children with intellectual disability (n = 297) and their closest in age siblings. The behavioral and emotional problems of the child with intellectual disability positively predicted sibling conflict over time. When accounting for control variables, this relationship was no longer present. Sibling warmth positively predicted the prosocial behaviors of the child with intellectual disability over time. When accounting for control variables, both sibling warmth and sibling conflict positively predicted the prosocial behaviors of the child with intellectual disability over time. Future research directions and clinical implications are discussed.