]I seem to recall from somewhere (L&T?) that the contraction "an't" was viewed very much like today's "ain't": a sign of low breeding. And in S&S, I had always associated this word with the Steele sisters.

Glad you brought this up. I circled this word in chapter 21 to bring up but somehow never got to it. I went off to do a little searching and from 2 online definitions it seems that this was used the way we use ain't today, now thought of as slang. I searched the text also and the word is used seven times in this novel:

Chapter 21 by Anne
Chapter 30 by Mrs. Jennings
Chapter 34 by Fanny
Chapter 35 by Lucy
Chapter 38 by Anne
Chapter 40 twice by Mrs. Jennings so Mrs. Jennings actually uses it the most.

]So I was astonished to discover that the first usage of this contraction comes, in chapter 34, from ... Fanny Dashwood! "They are very pretty, ma'am -- an't they?"

What's more astonishing is, if this is a word mainly associated with low breeding, is the fact is that she is speaking to Mrs. Ferrars when she uses it.

]Fanny Dashwood, whatever her other faults, never struck me as a particularly lower-class woman. Did I miss something? Or am I mistaken about "an't"?

When you consider the characters using this word, it would seem that you are on the right track to question this. Except for this one instance of Fanny using this word, it appears that it is as you say, a sign of a lack of breeding. For a second, it almost makes Fanny's usage of the word make her seem human, doesn't it? And she is actually giving Elinor a compliment while she is about it. Maybe it was a slip that wasn't caught in editing or maybe it was JA throwing a curve or possibly playing at some mischief, an inside joke that we don't get such as Fanny having been modeled after some real life character that gave JA pleasure to have her use this word. Perhaps others will have some thoughts on this.