It is notoriously difficult to identify peer effects within the family, because of the common shocks and reflection problems. We make use of a novel identification strategy and unique data in order to gain some purchase on this problem. We employ data from the universe of children born in Florida between 1994 and 2002 and in Denmark between 1990 and 2001, which we match to school and medical records. To address the identification problem, we examine the effects of having a sibling with a disability. Utilizing three-plus-child families, we employ a differences-in-differences research design which makes use of the fact that birth order influences the amount of time which a child spends in early childhood with their siblings, disabled or not. We observe consistentevidence in both locations that the second child in a family is differentially affected when the third child is disabled. We also provide evidence which suggests that the sibling spillovers are working at least in part through the relative exposure to parental time and financial resources.