A sister-in-law is your partner's sister, or your brother's wife. My sister-in-law.
The two meanings are quite different relationships and share one word. A sister-in-law is often the person who holds an extended family together.
1nounsister-in-lawYour partner's sister, or your brother's wife.
“His sister-in-law organised every family gathering for thirty years.”
sister-in-law is a relative.
Middle English suster-in-lawe; the law is church law about marriage.
Middle English
suster-in-lawe
English, today
sister-in-law
Middle English suster-in-lawe; equivalent to sister + in-law.
Etymology from Wiktionary, CC BY-SA