Sibling relationships provide unique social experiences that can vary across the lifespan. Early sibling social interactions (ESSI) have been associated with children’s own relationship and developmental outcomes, highlighting the essential role that sibling encounters play, even from a young age. Understanding how these social exchanges occur and unfold and the range of opportunities they provide can shed light on critical aspects of early childhood development and family life. However, the methodological approach used in studying ESSI can influence our understanding of these early experiences. This systematic review aims to delineate the methodological framework adopted in observational studies of ESSI. Through a systematic search of psychology and domain-general databases until March 2023, we focused on studies that addressed bidirectional naturalistic interactions in young sibling dyads (at least one child aged 0-36 months). Of the 713 articles screened, only 63 met the inclusion criteria. Findings regarding three main issues are examined, including sample characteristics, study designs and procedures, and sibling interactive behaviours targeted. Previous research has focused on a diverse range of sibling behavioral exchanges, including cues of children’s social skills and relationship quality within mainly ecological contexts. However, limitations in representativeness and standardization have been identified. Future studies should incorporate sequential analyses to fully comprehend the interactive nature of early sibling social encounters.