Sister is the common metaphor in English for an object or organization sharing the same origin or having a similar mutual close commonality: sister stations, Sister Cities, sister newspapers, sister schools, sister organizations, and so forth. The different branches of the armed forces are sister services (but brothers-in-arms). The metaphorical sense is included in every dictionary definition I checked.
Ships have been personified as females in Western languages for as long as anyone can remember. In Germanic languages, companion ships are known as sister ships-- schwesterschiff, zusterschip, systerskepp. (In the Romance world, she is a "twin," and the sister ship a navire-jumeau, nave gemella, etc.) I surmise this convention has been loosely extended, to other objects and organizations, most notably vessels but also countries, storms, and so on. Almost every poetic personification of a Western country is female: Brittania, Marianne, Columbia, Helvetia, Mother Svea, etc.
While referring to inanimate things as female is in decline, I do not perceive the use of sister for describe a pairing as out of fashion or frowned upon.