I've just realized, in reading the more recent posts, that for myself I mentally distinguish between saying "my brother's wife, Susan" and "Joe's wife, Susan." I wouldn't use the latter for anyone who didn't know that Joe is my brother without being told, and I wouldn't use the former for anyone who did.
So for a friend or acquaintance who has never met my brother and/or really doesn't know him, I'd introduce her as "my sister-in-law, Susan." If I felt it was necessary, I'd mention she was my brother Joe's wife after the introduction. For a friend or acquaintance who does know my brother on a first-name basis, it's possible I'd introduce her as "Joe's wife, Susan," but I might still just call her my sister-in-law. It would depend upon the context, I think, and what information I thought the person actually needed to know.
That's something else that occurs to me, now that I think of it. My pattern of introducing people is pretty habitually to say, "Jane, this is my sister-in-law, Susan. She's married to my brother Joe," and then say, "Susan, this is my friend Jane. She belongs to that book club I told you about." Basically, it's common for me to state the relationship and name in one sentence/phrase, and add some context in another (how I know them, more details about the relationship, etc.).