Siblings play crucial roles in each other's cognitive and language development because they observe, imitate, and receive guidance and support from each other during their interactions. In this research, we examined trilingual siblings (ages 6 and 9) from an immigrant family with a rarely explored cultural background (American-born children of Korean Chinese parents) and their multilingual interactions with a researcher. Employing cultural historical activity theory (CHAT) and Vygotsky's notion of mediation (Vygotsky, 1978), we particularly focused on the roles that the older multilingual sibling took on during interactions and what mediations of meaning-making took place that created learning opportunities. Our analysis was based on the interaction data between the siblings. Findings revealed that the older sibling played a unique role as a linguistic and cultural mediator, assisting meaning-maintaining processes in multilingual interactions by providing interlingual and intralingual interpretation and transferring linguistic and cultural knowledge. Given the current educational landscape that prioritizes learning English, and the complexities of maintaining three languages for trilingual children, this study adds to the literature highlighting the unique roles multilingual children can play in expanding others' languages, literacies, and cultures, as well as facilitating multilingual communicative events.