Social norms are critical for the effective functioning of social systems. Previous studies have mainly focused on the effects of in-group/out-group on moral violations and cultural differences in social norm violations. The impacts of such violations within intimate relationships remain poorly understood. In this study, 35 Chinese college students judged the appropriateness of their mother’s or a stranger’s behaviors (e.g., singing) in different situations (e.g., library). The participants’ electroencephalography activities were analyzed when mothers and strangers violated social norms or conformed to them, aiming to uncover the neural mechanism underlying how interpersonal intimacy influences the detection of others’ normative behavior. The ERP results revealed that compared with social norm conformity, social norm violations by the mother elicited a stronger N400 which was related to cognitive conflict, and strangers elicited a more positive LPP which was related to cognitive assessment. However, no significant difference was observed by strangers in N400 and by mothers in LPP between violating and conforming to social norms. These findings suggest that interpersonal intimacy influences the detection of social norm violations. These findings broadly support the Norm Focus Theory, which posits that individuals assess the behavior of those they are intimate with more quickly and strictly, while they exert more cognitive effort later when assessing a stranger’s violation behavior. This aligns with Chinese culture’s interpersonal tendencies, which emphasize self-discipline and the characteristics of ‘being round outside and square inside’. The results offer insights into how intimate relationships impact behavioral and neural responses to social norm violations.