On the other hand, I think it's weird that there's a term for it at all. For how often I need to refer to either concept, "brother's wife" and "wife's sister" a

On the other hand, I think it's weird that there's a term for it at all. For how often I need to refer to either concept, "brother's wife" and "wife's sister" are both easy enough to say. But it doesn't surprise me that we refer to then as people of the same generation; it's not much different from "step-brother" in that regard.