<p> Family is an important determinant of youth's health behaviors, yet research on the importance of intragenerational relationships for determining physical activity (PA) and sedentary behaviors (SBs) is limited. This study examined correlations in siblings' total PA, moderate-to-vigorous physical activity (MVPA), and SB and explored potential determinants of differences in Hispanic/Latino siblings' activity. Hispanic/Latino 8-16-year olds from the cross-sectional Hispanic Community Children's Health Study/Study of Latino Youth with ≥1 sibling enrolled were examined ( = 535). Activity was assessed using the Actical accelerometer. Linear mixed models were used with total PA, MVPA, or SB as the outcome; correlations among siblings' PA and SB were assessed with intraclass correlation coefficients (ICCs). ICCs for siblings' total PA, MVPA, and SB were 0.26 (95% confidence interval: 0.16-0.36), 0.29 (0.21-0.38), and 0.42 (0.34-0.51), respectively. There were no differences in correlations between siblings who were all brothers compared to all sisters. However, compared to siblings of differing sexes, siblings who were all brothers had the strongest correlations in total PA (0.61 [0.46-0.76]) and MVPA (0.64 [0.49-0.78]), and siblings who were all sisters had the weakest correlations in SB (0.14 [-0.10 to 0.37]). Correlations did not differ according to age, and social and environmental measures did not explain differences in siblings' PA or SB. Correlations in Hispanic/Latino siblings' PA and SB ranged from slight to fair, with siblings who were all brothers generally having the strongest correlations. Future research should examine determinants of sex-specific differences in siblings' PA and SB correlations.</p>