Unethical pro-family behavior is a prevalent, costly, and generally discreet employee activity in firms; however, there is limited research on the topic. Based on the self-classification theory, we collected three waves of data from employees of two firms in China to explore the influences of family-like employee-organizational relationship on unethical pro-family behavior (Study 1) and the boundary conditions between them (Study 2). The results of Study 1 showed that the family-like employee-organizational relationship had a significant negative impact on unethical pro-family behavior, with the relationship between the family-like employee-organizational relationship and unethical pro-family behavior mediated by both relational and transactional psychological contracts. Study 2 reconfirmed the findings of Study 1 and also found that differential leadership strengthened the positive impact of the family-like employee-organizational relationship on the relational psychological contract (from the perspective of 'insiders'). Overall, the findings of the study explain why, how, and when employees exhibit unethical pro-family behavior.