So why do people talk about mothers-in-law, sons-in-law, sisters-in-law, but not nephews-in-law, or aunts-in-law? It’s just nephews or aunts (or sometimes “my wife’s nephew” or the like). Naturally, practices doubtless differ in some measure, so some people might say “daughter” instead of “daughter-in-law.” But I think I’m right about the general pattern, and some quickie google searches support that.
Two possible theories:
We tend to be quite close to our parents, children, and siblings, so calling someone with whom we lack a blood tie — and whom we’ve often known only for several years — “mother,” “son,” or “sister” might grate on many people. That’s not as much of a problem with aunts or nephews.
We refer to uncles-in-law and nieces-in-law more rarely than to the other in-laws. Yet why would this stop us from just adding “-in-law” when we do need to refer to them that way? Maybe because given the rarity of the reference, the terms will always sound unusual and thus inapt. I’m not wild about this theory, but I just thought I’d flag it.
While we’re at it, why are there gender-generic terms “parents,” “siblings,” and “children” — and, in English, only a gender-generic “cousin” — but not “auncles” or “niecews”? (No, I’m not serious about those two particular suggestions; I’m noting the absence of any such word.)
UPDATE: Reader Stefani Smith correctly pointed out that I was using the word “blood tie” in item 1 loosely. I certainly meant to include adoptive relationships, where the emotional tie is generally as close as a blood tie, and step-relationships of long enough standing (for instance, a child’s relationship with a stepfather who had been parenting the child since a very young age). I was using blood tie to refer to the most common such longstanding, cohabitating, nuclear family relationship. But indeed the real point of my item 1 is that the value that people place on very close familial relationships (blood, adoptive, or longstanding step-) deters many from calling a mother-in-law simply “mother.”