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Sibling-in-law is a word phrase referring to the sibling of one's spouse, or the spouse of one's sibling, or (more rarely) taken one step further by referring to the spouse of one's spouse's sibling. All of these are relations which do not relate to the person directly by blood. Alternatively, the spouse of your spouse's sibling may be called a co-sibling-in-law, or co-sibling.
More commonly this is referred to by the gendered terms: brother-in-law, or co-brother-in-law for a male non-blood sibling, and sister-in-law or co-sister-in-law for a female non-blood sibling.
Just like other affines, or "in-laws", siblings-in-law are related by a type of kinship called affinity. Just like the children of one's siblings, the children of one's siblings-in-law are called simply nieces and nephews – if necessary, specified whether "by marriage", as opposed to "by blood" or "by adoption".
One study, examining the issue of envy in the triadic system of sibling, sibling-in-law and spouse, concluded that "The sibling-in-law relationship shared similarities with both spousal and sibling relationships" and that "Relational closeness and satisfaction for all relationships in the triad were correlated."
In Islamic law (shariʿa) and Jewish law (halakhah) sexual relations between siblings-in-law are prohibited as incestuous, unless the spouse is no longer married. Conversely, in Judaism there was the custom of yibbum, whereby a man had a non-obligatory duty to wed his deceased brother's childless widow so she might have progeny by him.
If one pair of siblings is married to another pair of siblings, the siblings-in-law are thus doubly related, each of the four both through one's spouse and through one's sibling, while the children of the two couples are double cousins.
|Look up sibling-in-law in Wiktionary, the free dictionary.|
- 1 Samuel 18:20–27.
- OED entry for 'brother-in-law' describes this as 'uncommon': 
- Cambridge Dictionaries Online. "Family: non-blood relations".
- Wiktionary, 'co-sibling-in-law': https://en.wiktionary.org/wiki/co-sibling-in-law
- Yoshimura, C.G (2010). "The experience and communication of envy among siblings, siblings-in-law, and spouses". Journal of Social and Personal Relationships.
- "Forbidden...that you should marry two sisters at one time"[Quran 4:23 (Translated by al-quran.info)]
- Leviticus 18:16, 18:18.
- Deuteronomy 25:5–10.