Associations of excessive gestational weight gain (GWG) with greater birthweight and childhood obesity may be confounded by shared familial environment or genetics. Sibling comparisons can minimize variation in these confounders because siblings grow up in similar environments and share the same genetic predisposition for weight gain. We identified 96 289 women with live births in 2008-2014 at Kaiser Permanente Northern California. Fifteen percent of women (N = 14 417) had at least two births during the study period for sibling analyses. We assessed associations of GWG according to the Institute of Medicine (IOM) recommendations with birthweight and obesity at age 3 years, using conventional analyses comparing outcomes between mothers and sibling analyses comparing outcomes within mothers, which control for stable within-family unmeasured confounders such as familial environment and genetics. We used generalized estimating-equations and fixed-effects models. In conventional analyses, GWG above the IOM recommendations was associated with 88% greater odds of large-for-gestational age birthweight [95% confidence interval (CI): 1.80, 1.97] and 30% greater odds of obesity at 3 years old (95% CI: 1.24, 1.37) compared with GWG within the IOM recommendations. In sibling analyses, GWG above the IOM recommendations was also associated with greater odds of large-for-gestational age [odds ratio (OR): 1.36; 95% CI: 1.20, 1.54], but was not associated with obesity at 3 years old (OR = 0.98; 95% CI: 0.84, 1.15). GWG likely has a direct impact on birthweight; however, shared environmental and lifestyle factors within families may play a larger role in determining early-childhood weight status and obesity risk than GWG.