Siblings are children’s most frequent daily companions in middle childhood and these relationships are theorized to be dynamic in nature but are rarely studied in a way that captures their day-to-day variation. Guided by cultural-ecological and relationship perspectives, this study examined the role of older siblings’ familism values and caring roles as predictors of average levels of and day-to-day variation in siblings’ positive and negative exchanges across seven days and tested moderation by child sex. Participants were older ( M = 10.6 years; SD = 0.39; 46.6% female) and younger siblings ( M = 8.2 years; SD = 1.07; 47.2% female) in 307 Latino sibling dyads recruited from 11 elementary schools in the southwestern U.S. Data were collected via home visits with parents, and in afterschool sessions and seven nightly phone calls with siblings. Analyses were conducted using a random variability model in a multilevel structural equation modeling approach. Findings revealed consistent associations between older siblings’ familism values and sibling roles and both average levels of and intraindividual variation in older siblings’ positivity and negativity. For younger siblings, some findings were moderated by child sex with more consistent links to younger sisters’ positivity and negativity than younger brothers’. Discussion highlights how older siblings’ cultural values and roles shape both overall levels of and day-to-day fluctuations in siblings’ exchanges and offers future directions of research.