The words we use for family members in English are specific about some things, and vague about others. Our vocabulary marks a distinction between our mother and her sisters (some languages use one word for mother and maternal aunts), but doesn't say whether siblings are older or younger (some languages have different words for brother and sister depending on their age relative to you). We lack words that pick out particular family members (we have "cousin," but what about child-of-my-father's-brother?) as well as certain general terms (we have "siblings" for brothers-and-sisters, but what about nieces-and-nephews?)
If you look hard enough, you can find some words to help fill in the gaps. Here are 11 unusual English kinship words for family members.
Child of your paternal uncle. Also, a child of your own brother. It hasn't gotten a lot of use in the past few centuries, but it was once convenient to have a term for this relationship because it factored into royal succession considerations. The first citation for it in the OED, from 1538, reads, "Efter his patruell deid withoutin contradictioun he wes king."
Your mother's brother. Latin distinguished between patruus, father's brother, and avunculus, mother's brother. (There was also amita, father's sister, and matertera, mother's sister.) It's the root of the word "avuncular," meaning "having to do with uncles" or "uncle-like" (i.e., kind and friendly, like an uncle). You won't find the word avuncle in the dictionary, but it has been used in anthropology texts and in papers concerning royal matters.
Your nieces and nephews. You won't find this in the dictionary either, but use of this term seems to be growing among favorite aunts and uncles who want an easy way to refer to their little bundles of sibling-provided joy in a collective or gender-neutral way.
Your father's sister. Latin amita covers this relationship, but we don't have to reach that far back to find an English equivalent. Old English made a distinction between aunts and uncles depending on whether they were maternal or paternal. We lost all that when we borrowed the more general "aunt" and "uncle" from French.
Your mother's sister. Old English
Your father's brother. Old English
Your mother's brother. It survived in some dialects as "eme," with a more general meaning of uncle or friend, into the 19th century.
Your half-brother from the same mother. A term used in old legal documents or other discussions of inheritance and succession. Half-siblings of the same mother are "uterine" and of the same father are "consanguine."
Full brother, sharing both parents. Nothing to do with Germany. The "german" here is related to "germane," which originally meant "of the same parents" and later came to mean just related or relevant.
10. Double cousin*
Full full cousin, sharing all four grandparents. Comes about when a pair of sisters marries a pair of brothers.
The parents of your child's spouse. Your child's in-laws. Ok, this is a Yiddish word, but one that, like a lot of Yiddish words, has poked its way into English because it fills a gap. When it comes to marriage, this can be a very important relationship, so it’s good to have a word for it. If your parents get along with their machetonim, the family—the whole mishpocheh—will be happier.
*Oops. Post originally had "Cousin-german" here. That's just a regular first cousin.