Brothers-in-law and sisters-in-law are bound by a type of kinship called affinity, like all in-laws relationships. All of these relationships are relationships that are not directly related to the person.  Generally used in translation or in countries like India (Andhra Pradesh) where the local language makes the distinction. In the general language of traditionally English-speaking countries, the sister-in-law can be used. Languages that have words for this relationship that are different from the word for sister-in-law. Not everyone is credited with the second meaning above, although this can sometimes be an oversight: I`m not a lawyer, but I don`t think many of the legal rights or duties are usually (or whatever the correct legal term is) attributed to the in-laws, so I don`t really see any logic behind this distinction. From co- + sister-in-law or co-sister + sister-in-law. I am sorry. I was wrong in my previous post. In Spanish, there seems to be a specific word to describe your sister-in-law`s husband. I was told that you can use “concuño” or “concuñado”. There is no equivalent specific word for this in English.
Definition 3 of a brother-in-law is the sister-in-law`s husband. Strictly speaking, the husband of a sister-in-law is simply called the husband`s brother-in-law, assuming that the sister-in-law in question is the husband`s sister. There is no direct relationship with the subject, a term used to describe the person or point of view when comparing family relationships. Usually, a brother-in-law or sister-in-law is the relationship that exists between a person`s brother and their spouse. This relationship is based on reciprocity, as it includes the relationship of siblings to spouses and from spouse to siblings. Most often, a brother or sister is called brother-in-law for a male brother and as a sister-in-law for a female brother or sister.  There is no word for this in Spanish or English. You can call him your “brother-in-law,” but your sister-in-law`s husband is technically not one of your parents at all. He is “the husband of your sister-in-law.” Brothers-in-law and sisters-in-law also refer to the reciprocal relationship between a person`s spouse and the spouse of his or her siblings. In Indian English, we can speak of co-brothers and sisters (specificity of a co-sister, for the wife of the brother-in-law, or co-brother, for the husband of the brother-in-law). 3) or the husband of the spouse`s siblings.
(Answer to initial question) Just like the children of his own siblings, the children of his brothers-in-law and sisters-in-law are simply called nieces and nephews – if necessary, whether “by marriage,” as opposed to “by blood” or “by adoption.” I have always understood that my sister`s husband is simply that and is in no way legally related to me, so not my brother-in-law, and if you knew my sister-in-law`s brother, you would not want to recognize him as a parent either. Let`s say that “A” and “B” are brother and sister and both are married. The wife of “A” is a sister-in-law of “B”, and “A” is a brother-in-law of the husband of “B”, but the wife of “A” and the husband of “B” are not related and have only a legal affinity with each other. Unless otherwise stated, there is therefore no legal relationship. While I respect Tosh and qfreed`s answers and acknowledge that we all – out of affection or courtesy – tend to call our in-laws our in-laws spouses our in-laws too, this is not technically/legally correct. The main use is for the wife of the husband`s brother. This rule is generally considered true in most English-speaking societies. However, in some parts of India and in some Indian languages, including English-speaking Indian societies, family relationships are defined in more detail.
There is a particular word invented for the relationship of the man who is married to the sister of a spouse: a co-brother or a co-brother-in-law. In many Indian languages, the word is shaddaka or sadoo and is used to refer to the oldest or husband of the spouse`s younger sister. There is no equivalent term for the wife of a brother or sister`s brother, or in other words, no co-sister or brother-in-law. I would like to see some sources say that these people are not brothers-in-law and sisters-in-law, except to express a personal opinion. I`ve seen errors in online definitions before, but someone cites a source that supports their explanation. By the way, some of us would never call our father-in-law our father. It is a concept of affection that must be earned. I call my father-in-law by his first name and I always call him “my father-in-law” or “my mother`s husband”. Never “father”.
A rarer use of the term is seen in “casual conversations” with the term brother-in-law describing the relationship between the brother-in-law: William`s brother, Charles, has a brother-in-law named James (James is Charles` brother-in-law here, not because he married his sister, but because he is a brother of Charles` wife), James referring to William as his brother-in-law.  www.avvo.com/legal-answers/legal-definition-of-brother-in-law-20578.html is the sister-in-law`s husband a brother-in-law? An in-laws relationship can only occur to one degree of the subject. A brother-in-law can only be the spouse of the brother or sister of the subject or the brother of the spouse of the subject. For this reason, the relationship is defined in relation to the spouse. However, if the sister-in-law is the wife of the brother or sister, her husband is the brother from the point of view of the subject. The original poster asked, “I thought he was brother-in-law?” For me, this means that she used the term “brother-in-law” or “cuñado” to refer to the husband of her husband`s sister, and she was told she was wrong, which caused her some confusion. .