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In some senses, co-sibling-in-law is marked as rare. Watch for register when choosing this word.
co-sibling-in-law
noun 2
Definitions
noun
1
One's spouse's sibling's spouse; either of two (or more) people who marry siblings, from the perspective of the other.
2
One's sibling's spouse's sibling; either of two (or more) people whose siblings marry each other, from the perspective of the other.
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