I was re-reading S&S for the GR and noticed a little detail previously overlooked. Colonel Brandon did not immediately fall for Marianne.

When he first saw Marianne when she was playing the piano, he was merely respectful of her playing performance. (Leave it to Mrs. Jennings to jump at conclusions.) It was in Chapter 10 that he really show the attachment, long after Mrs. Jennings and Sir John stopped teasing him about it. Elinor, however, noticed it at that point.