Supplementary education, which is different from the formal education system, is more likely to reflect the other form of inequality in educational opportunities through the process of education. In China, supplementary education is generally divided into two sorts: academic-oriented classes and hobby-oriented classes. Using the dataset of China Family Panel Studies 2018 and analysing with the Logit Regression models (LR), this study analyses how gender and sibling structure impact the students' supplementary education participation opportunities. Applying the "resource dilution theory" and "budget constraint theory" in Chinese modern families, siblings might have a competitive relationship with each other in obtaining limited family resources, while parents would assess the lifetime cost of raising a child when deciding the investment in supplementary education. According to the results, an increase in sibling numbers and density dilutes the family resources of supplementary education for each child on average. Females have more opportunities to participate in supplementary education than males, especially in hobby-oriented supplementary education. The "female advantage" is significant in the supplementary education opportunity of the first-born child and the "gender minority" child. This study claims that the family allocation of supplementary educational resources is influenced by economic motives. This study supports both the "resource dilution theory" and the "budget constraint theory" in supplementary education in contemporary China, while the issues of gender inequality regarding family resources allocation seem still remain.