The father's role in children's L1 maintenance and L2 learning is a relatively unexplored area. This study considers the L1 and L2 proficiency of 30 Korean-English late bilinguals who immigrated to New Zealand during their adolescence and how their L1 and L2 proficiency is influenced by the language use of family members. Data were collected through a questionnaire, a vocabulary test and a story-retelling task, and analysed in terms of language use and language proficiency measures. While language use of Korean siblings and fathers is shown to have a dual role in both affecting language use and L1 proficiency, the language use of Korean mothers is not associated with their children's patterns of language use or their children's L1 proficiency. We attribute the differences to different types of interaction between Korean mothers and fathers, and their adolescent children. The data also show that parental language use plays a minimal role in the adolescent L2 acquisition.