The present study examines the siblings’ contribution to shaping the language practices and language environment of immigrant families. The data consist of interviews, observations, and video recordings of everyday family interactions and sibling play in five Iranian families residing in Sweden. Detailed interaction analyses show that siblings targeted various aspects of each other’s language use (heritage languages, Swedish, and English). They corrected each other’s language use and choices and requested and provided language instructions when language-related problems occurred. However, the main language of siblings’ talk was Swedish. The siblings addressed the multifaceted linguistic demands of Swedish society and helped to develop each other’s multiple languages. Simultaneously, however, by primarily using Swedish, they were contributing to language shift. The study broadens the focus of family language policy studies (i.e., parents’ views on family language planning or parent-child dyadic interactions) and adds to the underresearched area of family bilingualism and heritage language maintenance.