|David and Jonathan became brothers-in-law when David married Jonathan's sister Michal|
Commonly a sibling-in-law is the relationship that exists between a person's sibling and the person's spouse. This relationship is reciprocal, as it includes relationship from sibling to spouse and from spouse to sibling. 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 one.
Less frequently sibling-in-law refers to the reciprocal relationship between a person's sibling and their spouse's sibling. In Indian English this can be referred to as a co-sibling (specificity a co-sister or co-brother).
Rarer usage of the term is seen in "casual conversation" with the term brother-in-law describing the relationship between one's brother's brother-in-law: William's brother Charles has a brother-in-law called James with William referring to James as being his brother-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 (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.
|Look up sibling-in-law in Wiktionary, the free dictionary.|
- Cambridge Dictionaries Online. "Family: non-blood relations".
- OED entry for 'brother-in-law' describes this as 'uncommon': 
- "Co-Sister". Cambridge Dictionary. Cambridge Dictionary. Retrieved 18 April 2020.
- "Co-Brother". Cambridge Dictionary. Cambridge Dictionary. Retrieved 18 April 2020.
- "English Language and Usage". site design / logo - Stack Exchange Inc. 2020. Retrieved 6 June 2020.
I'm afraid there is no commonly used name for that connection. Normally, you would say "my brother in law's brother". If you and William are very close, I suppose you could call him "my brother in law" in casual conversation, or just "a friend...Just say "brother-in-law" unless there is some reason that a more precise relationship is needed
- 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.