You would think that there are plenty of names for the varieties of family relationships: mother, father, daughter, son, sister, brother, the inclusive "sibling", husband, wife, all the in-laws, aunt, uncle, niece, nephew, cousins and first cousins and second cousins and first cousins once removed and double first cousins (and kissing cousins for the ones you can't remember the family tree for). But it turns out that there are vastly more relationships than we have names for.
What do you call the person you live with in a long-term relationship if you aren't married? Husband or wife implies marriage. Partner is common, but ambiguous due to its use in a lot of other fields. Lover and boyfriend carry connotations that may not be appropriate. Significant other is awkward. People in this situation are experimenting with lots of words, but none has risen to common usage yet.
What do I call my ex-husband's new wife, whom I like, when I am looking for a short-hand description of her position in my family? "My kids' stepmother" is slightly better than "my ex-husband's wife", but not much. "Sister wife" was suggested; I like the implication of familial affection but not the hint of polygamy. I'm stumped on this one, and I know I'm not alone in having the relationship, if not the desire to name it without invective.
Ok, fine, cohabitation and divorce/remarriage are relatively recent phenomena, at least at the scale that calls for new words. But even traditional extended families are given short-shrift in names for relationships. In The Stuff of Thought: Language as a Window Into Human Nature, Stephen Pinker points out that there is no collective gender-neutral
noun (like sibling) for the offspring of one's siblings (nieces and nephews, collectively), nor for the parents of
the spouse of one's children.
I guess the easiest solution is simple to call them all "kin", but that isn't very satisfying. With luck, the ever-inventive English language will find some new words soon.