<p>Families of children with an intellectual disability (ID) face several challenges that also have consequences for the well-being and mental health of these children's siblings. Many factors contribute to the siblings' well-being and mental health, and several studies have called for replicating their research with longitudinal data. In the present longitudinal study, the well-being and mental health of siblings of children with ID (the ID-group) or typical development (the comparison group) were investigated, considering family factors as mediating and moderating factors. Data were obtained from the Young Netherlands Twin Register, focusing on twins aged 5 to 12 years. Multilevel modelling was used to investigate effects on three sibling outcome measures: well-being, externalizing behavior, and internalizing behavior. The externalizing behavior of the index child (the child with ID in the ID-group; randomly chosen in the comparison group) mediated the relation between the intellectual status (ID vs. no ID) of the index child and their sibling's internalizing behavior. Index child externalizing behavior, parental well-being, and parental monitoring had significant effects on sibling outcomes. Maternal monitoring moderated the relation between the index child's externalizing behavior and their sibling's externalizing behavior. Thus, this longitudinal study confirms and nuances some previously reported relations concerning sibling outcomes. The results imply that it is important that care organizations use a family systems approach, especially when a child has externalizing behavior problems. This study shows that there are different ways to mitigate the impact of a child's externalizing behavior on their sibling, such as through improved parental monitoring.</p>