Previous research suggests that increased second language (L2) input at home may not support L2 acquisition in children from migrant backgrounds. In drawing this conclusion, existing work has largely aggregated across family members. This study contrasts the effect of L2 input from older siblings with that from mothers. Participants were 113 child L2 learners of English (mean age = 5;10 [range 4;10-7;2]; mean exposure to L2 in school = 16.7 months [range 2-48 months]). All children had at least one older sibling. Using hierarchical linear regression modelling with controls for age, non-verbal reasoning and phonological short-term memory, we found that greater L2 input from siblings -- but not mothers -- was associated with stronger L2 abilities in narrative macrostructure, inflectional morphology, and vocabulary. Increased cumulative exposure to the L2 at school and greater maternal L2 fluency were also positively related to children's L2 inflectional morphology and vocabulary scores.