Inequality in wealth has been well-documented, but its causes remain uncertain. Family processes in childhood are likely to shape adults' wealth accumulation, but these factors have attracted little attention. I argue that family size matters: children from larger families accumulate less wealth than do those from smaller families. Siblings dilute parents' finite financial resources and nonmaterial resources, such as time. This diminishment of resources reduces educational attainment, inter vivos transfers, and inheritance. Reduced educational attainment and transfers alter financial behavior; saving; and, ultimately, adults' wealth. I demonstrate that sibship size is associated with lower overall wealth in adulthood and that parents' resources and education, respondent's education, financial transfers, and financial behavior all mitigate the effect of siblings. Sibship size also reduces the likelihood of receiving a trust account or an inheritance and decreases home- and stock ownership. The findings provide important insights into early family processes that shape wealth accumulation and inequality.