Family structure and household dynamics influence early child development in a multitude of ways. This study examines how family size relates to infant development from 9 to 15 months. Development was quantified using the Bayley Scales of Infant and Toddler Development, Fourth Edition (Bayley-4) at both ages. This assessment provided a measure of cognitive, language, and motor development. Family environment and household data was reported by caregivers at both ages as well. This analysis focuses on whether family environment relates to differences in Bayley-4 scores at each age, as well as changes in performance over a 6-month interval. Previous research suggests that having an older sibling can both positively and negatively affect certain aspects of development. It is suggested that an older sibling may negatively affect language development, but positively affect motor development. Using a sample of 29 participants, it was found that having more older siblings (under 12 years old) was significantly correlated with lower language scores at both 9 and 15 months and lower cognitive scores at 9 months old. Motor scores were unaffected. The results from this analysis will provide insight into one of many factors that influences infant development.