Siblings of children with chronic disorders are at a heightened risk for internalizing and externalizing problems. Studies on sibling mental health typically use parent report, and sometimes sibling self-report, but do not include the perspectives of teachers. We investigated how teachers and parents in Norway rated sibling mental health to gain a more coherent picture of siblings' well-being and functioning across the home and school contexts. We compared how siblings aged 8-16 years (45% boys, 54% girls) were scored on the strengths and difficulties questionnaire by teachers (n = 125), mothers (n = 115), and fathers (n = 81) with population norms and clinical cutoffs. For boys, the mean teacher scores did not indicate problems for total difficulties or any subscale except hyperactivity-inattention, but mean parent scores were higher for total difficulties and most subscales (d = 0.44 to 0.96). For girls, teachers indicated higher than norm scores for total difficulties, emotional problems, hyperactivity-inattention and peer problems (d = 0.26 to 0.46), while parents indicated higher total difficulties and problems across most subscales (d = 0.31 to 0.54). Prosocial behavior was as a relative strength of siblings across all raters. Siblings may not display the same level of mental health problems in school as at home, and teachers can offer an important perspective on siblings' mental health and functioning.