There was an interesting post this week over at Language Log on the meaning of 'brother-in-law' and whether it can be used to describe the relationship between the husbands of two sisters, even though they themselves are not actually related at all. Several commentators felt that the term brother-in-law was fine to use this way.
I was going to comment that Russian has different words for the English brother-in-law, sister-in-law, mother-in-law etc depending on whether the relative was related to the man or the woman, but someone had already said the same thing (Karen, whose comment is 12.21am on November 20). I say "was related" as some of these words are dropping out of the Russian language, and many Russians don't know them these days.
Words survive in a language for as long as we need them, but when society or cultural practices change, the need for the words disappears. In a patriarchal society (as tsarist Russia was) the side of the family you were on -- the husband's or the wife's -- was significant and made a difference to your status within the family, hence the need for separate words.
When I was teaching English in Libya in the early 1980s, a student asked me one day for the name of his mother's brother in English (the relationship, he meant). I answered "He's your uncle". "No, he isn't", he replied, "my uncle is my father's brother". "Yes, it's the same thing", I continued, which he found absolutely impossible to comprehend because, of course, in that society, your father's brother and your mother's brother are not the same thing and do not have the same position in the family.
As I said, the different Russian words are dropping out of the language because the significance of being a paternal or maternal relative no longer has any relevance today. One of the first relationship terms to disappear -- you sometimes read it in 19th-century novels, but it's not even in dictionaries these days -- is yatrov' or yatrovka which is the wife of your husband's brother. That is clearly an important relationship in a patriarchal society -- two women who have married brothers and are therefore two outsiders, who perhaps feel a common bond with each other for this reason.