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From co- + sister-in-law or co-sister + -in-law.
Etymology from Wiktionary, CC BY-SA 4.0.
co-sister-in-law
Today it means: One's spouse's sister-in-law, especially one's husband's brother's wife, one's brother's wife in relation to the spouses of his siblings; either of two (or more) women who are married to brothers, in relation to the other. Full definition →