--- - |2 Family is an important context for cultural development, but little is known about the contributions of siblings. This study investigated whether older siblings’ cultural orientations and familism values predicted changes in younger siblings’ cultural orientations and familism values across 2 years and tested sibling characteristics and younger siblings’ modeling as moderators. Participants were 246 Mexican‐origin younger (Mage = 17.72; SD = 0.57) and older siblings (Mage = 20.65; SD = 1.57) and their parents. Findings revealed that older siblings’ Anglo orientations and familism values interacted with younger siblings’ modeling: When younger siblings reported high modeling, older siblings’ Anglo orientations and values predicted increases in younger siblings’ Anglo orientations and values. Discussion highlights the importance of siblings in cultural socialization. - Child Development, EarlyView.