We ask how a younger sibling’s physical disability affects sibling mental health using Danish register data on families with three or more children. Difference-in-difference models compare first- and second-born children when the third-born does or does not have a disability. Use of mental health services increases 13% among second-born children, with a 23% increase in psychiatric visits and an 18% increase in use of psychiatric medications. Effects are concentrated in households where the mother has less than high school and persist after conditioning on child test scores. Parental responses suggest that caregiving demands, stress, and strained resources are potential mechanisms.