Associations between effortful control and physical and mental health were examined among triads of mothers and twin children by simultaneously modeling the effects of one's own effortful control on one's own health (actor effects) with the effects of the other two family members' effortful control on one's health (partner effects). Families (N = 761 individuals; 254 families) included mothers (Mage = 39.98, SD = 5.64) and their twin children (Mage = 8.58, SD = 0.57; 51% female; 54.7% non-Hispanic White, 28% Hispanic). Mothers completed online and in-person questionnaires, and mothers' and children's body composition indicators and dominant-handgrip strength were directly assessed during two home visits. Actor effects dominated in families with twin boys and mixed-sex twins, whereas partner effects were more apparent in families with twin girls. In addition, the effects of children's effortful control on mothers' health depended on the sex of the twins. Findings uncovered important family dynamics linking effortful control to health, including that associations may vary by sex match of children in the family. (PsycInfo Database Record (c) 2022 APA, all rights reserved).