|Example of the spouse of one's sibling|
|David and Jonathan became brothers-in-law when David married Jonathan's sister Mary Anne|
More commonly, a sibling-in-law is referred to as a brother-in-law for a male sibling-in-law and a sister-in-law for a female sibling-in-law.
Sibling-in-law also refers to the reciprocal relationship between a person's spouse and their sibling's spouse. In Indian English this can be referred to as a co-sibling (specifically a co-sister, for the wife of one's sibling-in-law, or co-brother, for the husband of one's sibling-in-law).
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".
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.
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 (Sharia) and Jewish law (halakha), 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.
- Cambridge Dictionaries Online. "Family: non-blood relations".
- "Co-Sister". Cambridge Dictionary. Cambridge Dictionary. Retrieved 18 April 2020.
- "Co-Brother". Cambridge Dictionary. Cambridge Dictionary. Retrieved 18 April 2020.
- Yoshimura, C.G (2010). "The experience and communication of envy among siblings, siblings-in-law, and spouses". Journal of Social and Personal Relationships.
- Quran 4:23 : "˹Also˺ forbidden to you for marriage...two sisters together at the same time"
- Leviticus 18:16, 18:18.
- Deuteronomy 25:5–10.