We study whether childhood neighbourhood context affects mental health in adolescence in Finland. We also examine heterogeneous effects by family background. By exploiting register data for 1999–2018, we use sibling fixed effects models to gain more robust evidence on the existence of neighbourhood effects. We do not find evidence of an association between neighbourhood characteristics and psychiatric disorders within families. Differences in the effects by family background were not consistent, and variation was mainly found in random effects models. In general, observed family characteristics were strongly associated with psychiatric disorders. This means that interventions should be targeted to children at risk rather than certain neighbourhoods.