Combining 58 studies into a meta-analysis, Leonardi-Bee et al report that children in the U.K. are more likely to start smoking if both parents smoke (odds ratio 2.7), a brother or sister (OR 2.3), only their mother smokes (OR 2.2), or only their father (OR 1.7). If only their Aunt Haggie smokes, the effects are unknown. Authors strongly advise everyone in all families all quitting smoking, together, at once.

Leonardi-Bee et al. Exposure to parental and sibling smoking and the risk of smoking uptake in childhood and adolescence: a systematic review and meta-analysis. Thorax 2011;66:847-855.