Objectives In China, the recent replacement of the one‐child policy with a two‐child policy could potentially change family ecology—parents may switch investment from exclusively one child to two. The parent‐offspring conflict theory provides testable hypotheses concerning possible firstborn opposition toward further reproduction of their mother, and who wins the conflict. We tested the hypotheses that if there is any opposition, it will differ between sexes, weaken with offspring age and family resource availability, and affect maternal reproductive decision‐making. Methods Using survey data of 531 non‐pregnant mothers of only one child from Xi'an (China), logistic regression was used to examine effects of age, family income, and sex on the attitudes of firstborn children toward having a sibling; ordinal regression was used to investigate how such attitudes affect maternal intention to reproduce again. Results Firstborns' unsupportive attitude toward their mothers' further reproduction weakened with age and was overall more frequent in low‐income families. Sons' unsupportive tendency displayed a somewhat U‐shaped relationship, whereas daughters' weakened with family income; consequently, sons were more likely than daughters to be unsupportive in high‐income families, suggesting a tendency to be more demanding. Forty‐nine percent of mothers supported by their firstborns intended to reproduce again, whilst only 9% of mothers not supported by firstborns had such an intention. Conclusion Our study contributes to evolutionary literature on parent‐offspring conflict and its influence on female reproductive strategy in modern human societies, and has also important implications for understanding fertility patterns and conducting interventions in family conflict in China.