What? You can’t do it? You say English doesn’t have a word for that relationship?
You could, of course, insert a phrase rather than a word — “brother-in-law’s mother’s sister,” for instance, or just “sister’s aunt,” perhaps — but that’s not very satisfactory. If you pick any pair of languages, you’ll probably find some kinship term that is more specific in Language A than in Language B, and some other term that is more specific in Language B than in Language A. For example, what about a single word for “younger sister,” which English doesn’t have but Mandarin does? And you can see from the figure that Latin has two words for “uncle”: your mother’s brother is your avunculus, but your father’s brother is your patruus.
There’s plenty of literature on kinship terms in different languages — you can look it up with a little help from your friends, or at least from Mr. Google — but that’s not what I want to describe today. Right now I want to talk about what I recently learned about kinship terms in English. In particular, of course, some kinship terms I hadn’t known before and maybe you don’t know either. These come from a fascinating recent article in Mental Floss by the excellent linguistics popularizer Arika Okrent; I’ll write a bit about it, but you should really follow the link and read the entire article yourself. It’s not very long.
I’ve already given away the first two terms that Okrent mentions, except that I lied when I said that those two words for “uncle” are not found in English. It turns out — surprise! — that there are English words derived from the Latin terms, namely patruel and avuncle. So there! Okrent also reminded me about niblings and machetonim, terms already familiar to me from childhood, especially machetonim, one of my father’s favorite words. New to me were fadu, modrige, foedra, eam, brother-uterine, brother-german, and double cousin. Read the article to find out what they mean, as well the etymologies of all eleven!