This study aimed to illuminate the implications of COVID-19 school closures for sibling dynamics among Latinx school-age children in the US and to examine family and cultural factors that may have conditioned school closure effects. Data came from an ongoing study of Latinx families in Arizona that collected home visit and survey data prepandemic (fall 2019; T1) and daily diary data during the outbreak (February to May 2020; T2). The analyses focused on 215 Latinx children (47% female; M age= 9.72, SD= 1.22; 88% Mexican-origin) from 116 families (T1 family income median= $27,600, SD= $24,421). Multilevel tobit regression models were estimated to examine associations linking both T2 school closure and number of days since school closure with daily sibling positivity and negativity. The models also tested moderation effects of T1 family socioeconomic status, sibship size, child enculturation, and prior sibling positivity and negativity on these associations. Results showed that, although main effects of school closure on sibling dynamics were nonsignificant, school closure was linked to more sibling positivity in families with more children and among more enculturated children, and days since school closure was linked to more sibling positivity in families with more children and to lower sibling negativity among those with less prepandemic sibling negativity. Findings highlight differentiated effects of COVID-19 school closures on sibling dynamics among Latinx children whose families have been disproportionally affected by the COVID-19 pandemic, illuminate the role of Latinx family cultural strengths, and have implications for intervention programs and public policies.