Longitudinal data were obtained from 157 children aged 3 years 3 months to 5 years 6 months at Time 1. At Time 2 these children had aged an average of 12 months. Theory of mind (ToM) and executive functioning (EF) were measured at both time points. Results suggest that Time 1 ToM scores predict Time 2 EF scores. Detailed examination of sibling influences suggests that benefits—in terms of advanced ToM development—accrue to children with siblings versus without, and to those with a larger number of child-aged siblings. Any advance in either area (ToM or EF) is likely to benefit the other, and early sibling interaction appears to act as a catalyst.