<p>Siblings of children with chronic disorders are at risk of mental health problems, with risk factors including both individual-level characteristics and home-based, family-level factors. Such risk factors tend to inter-correlate, making non-family perspectives, such as those of teachers, an important part of a multi-informant approach to assessing sibling mental health. We investigated whether sibling-, mother- and father-reported family-level factors were associated with teacher-rated mental health of siblings in Norway. We used data from 127 teachers reporting on siblings’ mental health (sibling <i>M</i>_age = 10.3 years, <i>SD</i> = 2.0, 53.4% girls, 46.5% boys) to examine associations with familial factors reported by 113 mothers, 85 fathers, and 118 siblings of children with chronic disorders. We found that sibling-reported difficulties with adjustment to living with their brother or sister with a chronic disorder was moderately associated with more teacher-rated internalizing problems for siblings. Lower father-reported family social support was moderately associated with more teacher-rated externalizing problems for siblings. These associations remained significant in multivariate analyses examining all factors simultaneously as predictors of teacher-rated mental health and controlling for sibling age and sex. Emotional and behavioral problems of the child with a chronic disorder, parental mental health, and age difference between the sibling and the child with the disorder were not associated with teacher-rated mental health. Our findings suggest that teachers can detect mental health difficulties in siblings and therefore can be valuable informants on sibling mental health as part of a broader multi-informant approach.</p>