The main aim of this study is to build a new observational coding system for sibling interactions (Sistema de Codificación de Interacciones entre Hermanos -- SCIH) and to analyze its psychometric properties and validity. First, this study offers the first Spanish adaptation of a self-report questionnaire to measure sibling relationships that replicate the original structure of the SRQ in a sample of 469 children and adolescents. Then, the study focused on the development of the SCIH, which analyzes siblings' relationships by means of a system of live observation and it examined five dimensions: Warmth, Conflict, Dominance, Aggression, and Support in a sample of 50 pairs of siblings. The main contribution of this study is the description of the development of a new observational coding system for siblings and the confirmation of its inter-rater reliability. Limitations and future lines are discussed.