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Firstborn children have higher prevalence of obesity than secondborn siblings. The birth of a sibling typically results in resource dilution when mothers begin to divide their time and attention between two children. This mixed-methods analysis applies the family systems process of resource dilution to test the hypothesis that characteristics of the secondborn impact how parents feed the firstborn. Participants (n = 76) were mothers of consecutively born firstborn and secondborn siblings who participated in the INSIGHT trial and an observational cohort. Quantitative analyses involved multilevel models to test if characteristics of secondborns (temperament at 16 weeks, appetite at 28 weeks) were associated with maternal feeding practices of firstborns (structure and control-based feeding) at 1, 2, and 3 years, adjusting for firstborn child characteristics. A purposive subsample (n = 30) of mothers participated in semi-structured interviews to contextualize potential sibling influences on maternal feeding practices during infancy and toddlerhood. Quantitative data showed secondborn temperament and appetite were associated with how mothers fed their firstborn. Qualitative data explained maternal feeding practices in three primary ways: 1) Mothers explained shifting predictable meal and snack routines after birth of the secondborn, but did not perceive sibling characteristics as the source; 2) Family chaos following the secondborn's birth led to "survival mode" in feeding; and 3) Social support was protective against feeding resource dilution. The family systems process of resource dilution is a focus for future research and support for families during key transitions and a direction for efforts to reduce risk for child obesity.