noun
sister-city relationship
Refers to the formal affiliation between two cities, often involving cultural or economic exchange.
両市は姉妹友好都市関係を結んでいる。
The two cities have a sister-city relationship.
姉妹都市 is the more common term for 'sister city'; 姉妹友好都市関係 is a longer, more formal expression emphasizing the friendly relationship.
Compound of 姉妹 (sisters), 友好 (friendship), 都市 (city), and 関係 (relationship).