隔一場婚禮：最常見的兩條線
家族樹上只要畫一條婚姻線，這個女生就到位了：不是你結婚把她帶進來，就是你兄弟結婚把她帶進來。日常說 sister-in-law，多半是這兩種。
noun · B2
The sister of the person you are married to. Your husband's sister, or your wife's sister, is your sister-in-law.
大姑／小姑／姨子
核心畫面
One way to hold this word: picture your family tree with two kinds of line on it. Some lines were drawn at birth; some were drawn at a wedding. A sister-in-law is a woman standing where a sister would stand — same generation as you, close enough to sit at the family table — but the line that put her there is a marriage. The word never says whose wedding it was, so the same three cover your husband's sister, your 's wife, and the woman who married your husband's . That silence is why English speakers so often add one short sentence afterwards to say which line they mean.
可以想成一張家族樹，上面有兩種線：一種是血緣畫的，一種是婚禮畫的。sister-in-law 站的位置就是「姊妹」那…更多收起
可以想成一張家族樹，上面有兩種線：一種是血緣畫的，一種是婚禮畫的。sister-in-law 站的位置就是「姊妹」那一格，年紀同輩、坐同一桌吃飯，但把她放進去的是一場婚禮。這個字只說「同輩、女性、因婚姻而來」，從來不說是誰的婚禮，所以配偶的姊妹、兄弟的太太、妯娌，全部共用同一個字。中文剛好相反：大姑、小姑、姨子、嫂嫂、弟媳、妯娌，一開口就把線路和輩分標死了。所以學這個字的重點，是習慣英文的作法——先用 sister-in-law 把人歸位，再補一句說明是哪一條線。From the word’s history
Middle English suster-in-lawe: sister plus -in-law, literally a sister by law rather than by birth. The -in-law half is exactly what the modern core image keeps: the line on the family tree was drawn by a marriage.
中古英文寫作 suster-in-lawe，就是 sister 加上 -in-law，字面上是「法律上的姊妹」。今天的核心畫面留下的正是 -in-law 這半邊：家族樹上那條線是婚禮畫的，不是血緣畫的。同一組還有 mother-in-law、daughter-in-law。
Wiktionary, entry 'sister-in-law' (kaikki:3f6f69a190036ae4:115955229:137101): 'Inherited from Middle English suster-in-lawe; equivalent to sister + -in-law.'
Two branches
隔兩場婚禮：妯娌
同一張家族樹，只是要走兩條婚姻線才接得上：你嫁進一個家，她也嫁進同一個家。位置還是同輩女性，所以英文沒有換字。
noun · B2
The woman who is married to your husband's brother or your wife's brother. When two women marry two brothers, each one is the other's sister-in-law.
妯娌