The development of social relationships influences a person's self-concept, which in turn affects their perceptions and neural correlates in social interactions. This study employed an EEG-based hyperscanning technique and a longitudinal design to investigate how the evolution of interpersonal relationships impacts inter-brain synchrony during nonverbal social-emotional interactions. The framework for this study is based on the self-expansion model. We found that dyads exhibited enhanced affective sharing abilities and increased brain-to-brain synchrony, particularly in the gamma rhythm across the frontal, parietal, and left temporoparietal regions, after seven months together compared to when they first met. Additionally, the results indicate that inter-brain coupling evolves as relationships develop, with synchrony in nonverbal social-emotional interactions increasing as self-expansion progresses. Crucially, in the deep learning model, interpersonal closeness can be successfully classified by inter-brain synchrony during emotional-social interactions. The longitudinal EEG-hyperscanning design of our study allows for capturing dynamic changes over time, offering new insights into the neurobiological foundations of social interaction and the potential of neural synchrony as a biomarker for relationship dynamics.