The present study examined the effects of socioeconomic status (SES) and sibling differences in birth weight on sibling differences in the receipt of maternal sensitivity (i.e., differential parenting). It was hypothesized that sibling differences in birth weight would predict "absolute" differential parenting across the sibship (i.e., the more different siblings' birth weight, the more different the level of sensitivity in the family, overall) and "child-specific" differential parenting (i.e., relatively heavier siblings receiving more sensitivity, compared to his or her counterpart within the family). It was also hypothesized that there would be greater sibling differences in birth weight in lower SES settings. Multiparous mothers were recruited within two weeks of childbirth and filmed interacting with each of their children when younger siblings were 1.60 years (SD = 0.16, N = 396 younger siblings) and next-older siblings were 4.05 (SD = 0.75; N = 396 older siblings). Videotapes were coded for maternal sensitivity. Multilevel path-analysis revealed that lower-SES families exhibited greater sibling differences in birth weight, which corresponded to greater absolute differential parenting. Also, heavier siblings received relatively higher levels of sensitivity within the family. This study demonstrates that child and contextual factors operate together in predicting differential parenting.