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.
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.'
登入就可以看每一支怎麼連回核心畫面。