I don't disagree here. I'm commenting that the terminology dates back to familial structures and housing arrangements that largely no longer exist. Hence, arch

I don't disagree here. I'm commenting that the terminology dates back to familial structures and housing arrangements that largely no longer exist. Hence, archaic (since it's a historical artifact). As alluded to by other commenters: in China, women would be "married out" such that they would live with their husbands' families. As a result, the notion of "younger sister from outside" versus "younger sister down the hall" arose. That distinction doesn't exist if the newlywed couple doesn't move in with either of their parents. ... On the note of your question: English also has husband/wife, widow/widower, fiancé/fiancée (okay those are technically French loanwords). In the husband/wife scenario, we might use "spouse". And I still find "partner" a bit odd when it's used to refer to heterosexual couples, even if it's perfectly applicable. Is the more specialized terminology entirely irrelevant in either case (English or Chinese)? No, it conveys more information. But there are certainly situations where it's just not applicable (e.g. if one or both entities identify as nonbinary). (and as an aside, it's a distinction that makes the language harder to learn with all of its nuances -- my father has no sisters, so I grew up assuming that it was just "which parent's side is the cousin on?")