sibling-in-lawn.
/ˈsɪblɪŋɪnˌlɔː/, U.S. /ˈsɪblɪŋᵻnˌlɔ/, /ˈsɪblɪŋᵻnˌlɑ/ A person who is a sibling by marriage; the sibling of one's spouse or partner, or the spouse or partner of one's sibling.Originally in Anthropology; now sometimes as a gender-neutral alternative to brother-in-law n., sister-in-law n.
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1918 Amer. Anthropologist 20 416 (table) [Kinship terms of Kootenay Indians] Parent in law..child-in-law (after spouse's or child's death)..sibling-in-law (after spouse's or sibling's death) [etc.].
1984 Fort Madison (Iowa) Daily Democrat 6 Feb. 5/3 The gender words..should be changed, the project recommended... Sister-in-law would become sibling-in-law,..watchman would become security guard.
2017 Philadelphia Inquirer (Nexis) 1 Dec. You can imagine my panic when I learned my parents-in-law are moving across the street from us... One of my siblings-in-law may move to the neighborhood, as well.
This is a new entry (OED Third Edition, June 2022).
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n.1918