This study analyzes the legal relationship between customary law and Islamic law within the cultural taboos of the Banjar society, specifically the prohibition of simultaneous sibling weddings (pantangan kawin beimbai saudara). Begins by identifying its position within customary law, exploring its legal rationale alongside the Islamization process, and applying intersection theories between customary and Islamic law to establish a coherent synthesis. Utilizing normative legal research, this study examines legal materials through a literature review of relevant texts, complemented by insights from key informants. The findings clarify that this practice, also known as ‘bealahan’, originally carried mystical connotations across generations. However, through the Islamization process, it is revealed to possess a rational foundation: its objective is to prevent direct or latent conflicts arising from disparities or forced compromises between siblings in a marriage. Analyzed through the Receptio in Complexu theory, aligning the practice with its foundational objective provides a justifying rationale alongside Islamization efforts. Legally, this custom aims for maslahah and the prevention of mudarat, corresponding with the aspirations of Islamic law, where Islam is fully accepted and applied by its adherents, including within their customs. Furthermore, when linked to the Receptio a Contrario theory, this prohibition constitutes an acceptance, provided its existence and objectives do not contradict Islamic law. Within Islamic law, although this custom involves the concept of pamali, its examination and Islamization show that it contains values of maslahat and does not oppose religious principles. Consequently, it is acceptable as 'urf shahih and a component of maslahah.