Background. Due to shared health behaviors and disease risk, families may be more effective targets for health promotion. This study assessed whether providing family health history (FHH)–based risk information for heart disease and diabetes affected encouragement to engage in physical activity (PA) and healthy weight (HW) maintenance and co-engagement in physical activity among 320 Mexican-origin parents and their 1,081 children. Method. At baseline and 10 months, parents indicated who they encouraged and who encouraged them to engage in PA/HW, and with whom they co-engaged in PA. Households were randomized to receive FHH-based assessments either by one or all adult household members. Primary analyses consisted of regression analyses using generalized estimating equations. Results. At baseline, parents reported encouraging their child for both PA and HW in 37.6% of parent–child dyads and reported receiving children’s encouragement for both in 12.1% of dyads. These increased to 56.8% and 17.5% at 10 months (p < .001). Co-engagement in PA increased from 11.4% to 15.7% (p < .001), with younger children (30.4%) and mother–daughter dyads (26.8%) most likely to co-engage at 10 months. Providing FHH-based risk information to all adult household members (vs. one) was associated with increased parent-to-child encouragement of PA/HW (p = .011) at 10 months but not child-to-parent encouragement. New encouragement from parent-to-child (p = .048) and from child-to-parent (p = .003) predicted new 10-month PA co-engagement. Discussion. Providing FHH information on a household level can promote parental encouragement for PA/HW, which can promote greater parent–child co-engagement in PA. In this high-risk population with a cultural emphasis on family ties, using FHH-based risk information for all adult household members may be a promising avenue to promote PA.