This study examined the effects of older siblings' L1/L2 use on the vocabulary and fast mapping skills in preschool-age children who learn Cantonese (L1) as a home language and start to learn English (L2) in preschool settings. Two groups of bilingual children participated in this study: (1) children whose older siblings used mostly L1 at home ('Mostly L1' group; n = 14) and (2) children whose older siblings used an equal amount of L1 and L2 at home ('Equal L1/L2' group; n = 13). Participants were tested using vocabulary measures and a fast mapping task. Results showed that there were no significant group differences in children's vocabulary scores or in their fast mapping production scores in each language. However, when measured using a comprehension probe during the fast mapping task, the 'Mostly L1' group outperformed the 'Equal L1/L2' group in identifying the target novel words in L1, whereas there were no group differences in L2. The findings provide some preliminary evidence that older siblings who speak mostly L1 may contribute to their younger siblings' L1 fast mapping skills.