This study compares Rafsanjan and Gaziantep (pistachio producers) to propose a 'product-oriented sister city' model based on urban diplomacy and agritourism for sustainable development of medium-sized cities. The research employs a qualitative meta-synthesis method, with systematic analysis of 40 reputable scientific sources and conceptual coding. Despite its agricultural and cultural advantages, Rafsanjan faces challenges such as sanctions and financial constraints, environmental crises (water scarcity and land subsidence), and institutional weaknesses in branding. In contrast, Gaziantep has strengthened its global identity through membership in the UNESCO Creative Cities Network, international festivals, and food tourism infrastructure. Based on the similarities between the two cities in pistachio-based economies, food culture, and agricultural traditions, the "Pistachio Sister Cities" initiative is proposed. The conceptual model derived suggests that the success of urban diplomacy depends on the synergy of three components: "institutional-diplomatic," "branding and tourism infrastructure," and "experience-based economy," which are reinforced through the mediating variable of "transnational sisterhood networking." The findings indicate that Gaziantep scores highly across all three components of the model, particularly in the institutional-diplomatic and branding dimensions. In contrast, Rafsanjan, despite its production advantages, faces deficiencies in all three components, with the lack of structured strategies, institutional weaknesses, and environmental crises identified as the most significant obstacles to urban development. Based on the innovative "Pistachio Sister Cities" model proposed as a cooperation framework, its realization requires the design of alternative pathways (informal scientific-cultural collaborations, virtual public diplomacy, private sector participation, and multi-level support from national and provincial institutions) and gradual management of obstacles (with emphasis on environmental carrying capacity and sustainable water resources). The proposed model, while offering operational applicability for Rafsanjan, can be generalized as a methodology for other medium-sized cities with distinctive agricultural products, and can provide a roadmap for transformation from a "mere producer" to an "international symbol."