Indians especially have a tendency to use the terms co-son-in-law and co-sister-in-law while referring to relations.

  1. The husband of one's wife's sister is called Co-son-in-law.
  2. The wife of one's husband's brother is called co-sister-in-law.

While introducing the relations to others they are called so.

The terms may be understood in the Indian sub-continent. But such terms are not understood by the native speakers.

I searched on the internet for the equivalent terms in English but I could not find them.

I here with attach two links so that you may understand what I mean:

“Co-brother” or “co son-in-law?”


What are the equivalent terms used by the native speakers while introducing them (the mentioned relatives) to others?


4 Answers 4


In US English (and likely most non-India regions), we refer to these persons as simply brother-in-law or sister-in-law. My wife has a sister, and that sister is married. I refer to both husband and wife as my brother and sister-in-law.

In the same way, my mother has a brother who is married. I refer to both husband and wife as my aunt and uncle. In both cases the spouse becomes referred to as the same relationship. We would know from the social context that "aunt and uncle" refer to wife and husband rather than brother and sister, and we would therefore know that only one is a blood relation. In the same way, when "brother and sister-in-law" refer to a married couple, we know the actual relationships.

EDIT: The expression "co-son-in-law" would seem to contradict US English usage. Brother/sister-in-law refers to someone of the same generation as yourself. Son/daughter-in-law refers to someone of the same generation as your child and NOT the same generation as yourself. My daughter's husband is my son-in-law, so I would have expected co-son-in-law to mean something similar. But I gather it does not!

SECOND EDIT: It's clear from the surrounding discussions that regional usage varies. There are differences depending whether the speaker's intent is to identify a person, a married couple, or a blood relationship. My own usage (which, by definition, is regional) is not the same as what's familiar usage to others here.

  • 2
    Please, please, please: in most varieties of English.
    – Lambie
    Commented Sep 2, 2019 at 17:19
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    I don't think co-son-in-law contradicts American English. It seems that co-son-in-law references the parents-in-law. I am my wife's parents' son-in-law. Her sister's husband is also the son-in-law of my wife's parents. We are both sons-in-law of the same couple, thus co-sons-in-law (of our wives' parents).
    – Juhasz
    Commented Sep 3, 2019 at 15:57
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    I think it's a typo in the question; Wiktionary indeed defines "co-brother-in-law" as the wife's sister's husband.
    – chepner
    Commented Sep 3, 2019 at 18:11
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    @EdwardBarnard, when you say "I refer to both husband and wife as my brother and sister-in-law," do you mean "brother and sister-in-law" as you have written, or "brother- and sister-in-law," which indicates that the male is a brother-in-law?
    – shoover
    Commented Sep 4, 2019 at 3:49
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    That was a grammar quibble. "brother and sister-in-law" means (brother) and (sister-in-law). "brother- and sister-in-law" means (brother-in-law) and (sister-in-law).
    – shoover
    Commented Sep 6, 2019 at 1:59
  1. The husband of one's wife's sister is called Co-son-in-law.

  2. The wife of one's husband's brother is called co-sister-in-law.

In the United States, the husband of your wife's sister is called your "brother-in-law". Note that we use the same term for the husband of your own sister.

The wife of your husband's brother is you "sister-in-law". This is the same term we use for the wife of your own brother.

I had a conversation once with a speaker of some Indian language — Hindi maybe? — who said that his native language had many more different words for different relationships that in English we use the same word for. Like he said that in his language there were separate words for your father's mother and your mother's mother, while in American English we call them both "grandmother".

When we need to distinguish, we describe the relationship. Like if I said, "George is my brother-in-law", someone might ask, "Do you mean your sister's husband or your wife's brother?"

  • You can also qualify a relationship, e.g. Grandmother/Grandfather could be paternal or maternal to indicate whether your mother's parent or father's parent i.e. 'paternal grandmother'. My 'brother-in-law' is by definition married to my sister. However it's correct that we don't normally differentiate Husband of wife's sister as technically he wouldn't have any relationship to me: informally however such a relationship may also be described as brother-in-law.
    – charmer
    Commented Sep 3, 2019 at 13:09
  • @aniline suggested I change my last line from "sister's husband's brother" to "wife's brother". I accepted that edit as I think it is a better and more common example. But I think my original was valid. :-)
    – Jay
    Commented Sep 3, 2019 at 16:30
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    @charmer According to thefreedictionary.com, thefreedictionary.com/brother-in-law, a brother-in-law is, 1. the brother of one's spouse, 2. the husband of one's sister, or 3. the husband of a sibling of one's spouse. All 3 of the standard dictionaries that they cite list all 3 relationships. I don't think it's valid to say that only the second is the "real" definition and the other 2 are "informal".
    – Jay
    Commented Sep 3, 2019 at 16:37
  • Jay, sure you are entitled to your opinion - however 1 and especially 3 are absolutely no legal relation - hence informal, and there's nothing wrong with informal either.
    – charmer
    Commented Sep 4, 2019 at 15:13
  • @charmer Well it's not "my" opinion, it's the opinion of 3 standard dictionaries. I don't think there is any universal legal definition of "brother-in-law", and even if there was, a definition for the purpose of law is not the same as the general definition. Like many laws define "person" to include a corporation, but we do not think of corporations as people in normal conversation. According to Black's Law Dictionary, a brother-in-law, even by the definition you prefer, is not a relative. thelawdictionary.org/brother-in-law Another legal site I found gives the same 3 definitions ...
    – Jay
    Commented Sep 4, 2019 at 15:46

I would like to answer not because I did not understand what you have explained but because I think it is the best occassion to share our cultural differences or even heritage.

I belong to the south of India and my mother tongue is Telugu.

I have three brothers and one sister and I am married.

I call my wife's sister's husband as younger brother and he calls me elder brother because I am the elder of the two. But while introducing to others we call each other co-son in law such as He is my co-son-in-law..

Regarding my brothers' wives we call them sisters-in-law.

My wife's brothers are addressed as brothers-in-law.My sister's husband is also called in the same terms

My mother's brother is called maternal uncle and my father's brother is called paternal uncle. My wife's father is called Father-in-law in English.Of course there are different terms in our mother-tongue for the said relations.

We call the mother of father and mother as grand mothers (paternal grand mother and maternal grand mother) in English but they are called in the sense of mother's mother and father's mother in our mother-tongue.

The difference seems to be the usage of co-sister-in-law and the co-brother-in-law which are not used in native English speaking countries.

I thank those two Americans who were kind enough to answer very promptly

  • 3
    I sincerely appreciate your sharing this perspective with us. English is quite limited in expressing family relationships, so it makes sense that concepts would be brought over from the mother-tongue. I had no idea of the situation with regard to southern India English. Commented Sep 2, 2019 at 21:00
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    "I belong to the south of India". That strikes native English speakers as... odd, and stirs visions of nativism. "I come from the south of India" is what people would say.
    – RonJohn
    Commented Sep 3, 2019 at 0:45
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    @EdwardBarnard even southern Indian English, this isn't universal. It probably depends on how important exactness of relationships are to the local culture, I suppose. (I have lived in various parts of western, southern and eastern India. My mother tongue is Malayalam, from a state to the southwest of Jagatha's. This is the first time I'm hearing of any "co-<...>-in-laws")
    – muru
    Commented Sep 4, 2019 at 1:45

Contrary to the other answers I would never refer to someone who wasn't married to my sibling, or the sibling of my spouse, as a brother or sister in law. The relationship asked about here is not one that has a specific term. I would refer to such a person as "my sister-in-law's brother" as an example.

I'm a native English speaker from Ireland.

  • Same from a native Canadian (Quebec) English speaker. I spent 34 years in Canada and now nearly 20 in the US (New England and Texas).
    – Flydog57
    Commented Sep 3, 2019 at 14:06
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    You misunderstood the question. The question is asking about someone married to your spouse's sibling. (e.g. My wife's sister's husband) Commented Sep 3, 2019 at 16:21

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