Not brother and sister!
Earlier in the film, Emma and Mr. Knightley had joked about being like a brother and sister.
Then as they step outside to talk during the dance at the Westons', they really warm up to each other. He asks her with whom she will dance, and she answers,
"With you if you will ask me ... after all, we are not brother and sister!"
Mr. Knightley laughs at this, but, as he intently watches her walk away, says to himself in almost a purr, "indeed, we are not!"
Oh, and in the dance that follows, I could just swoon. What a great way to build romance!