The family, as the smallest and most basic social institution, has always been the focus of religions, thinkers, and schools of thought. The foundation of this institution is formed in the form of the first committed social bond, namely marriage, and acts as a source of production and utilization of social capital. The relationships between siblings, along with interactions with other family members, have extensive emotional and educational dimensions. Due to the importance of the type of sibling relationships in the formation of family social capital, it is necessary to examine sibling relationships from the perspective of the Quran and narrations with a social capital approach. This research aims to explain the duties of siblings towards each other, within the framework of social capital theory and with a descriptive-analytical method, and examines Islamic teachings in this area. The central argument of the article is that Islamic teachings, by designing a coherent system of rights and duties, transform sibling relationships into powerful and sustainable social capital in the family institution. The research findings showed that these duties are in three areas of personal privacy, including the rules of privacy and marriage, gaze and covering; The area of financial responsibilities, including wills, inheritance, priority in carrying out financial duties, and the scope of sibling interactions, reinforces the three main components of social capital theory: connection, norms, and public trust.