Wonderful foreshadowing, here!

Also, very clever insofar as how characters drive the plot is concerned. Because let's face it; had Mrs C been a ravishing beauty, Elizabeth would've rejected her from the get-go, a) because Mrs C would outshine her, and Elizabeth wanted an "inferior" companion to whom she could feel superior; and b) because the "danger" would have been a lot more obvious.

That sentence is worth reading more closely again: remember, that bit about "freckles, and a projecting tooth, and a clumsy wrist" is an indirect quote from Sir W, who, Anne is thinking, "was continually making severe remarks upon [those features] in [Mrs C's] absence" -- however, methinks Sir W doth protest too much in an attempt to pull the wool over his daughters' eyes, because in the very next breath, Anne is thinking, "but [Mrs C] was young, and certainly altogether well-looking". In other words, Sir W is nit-picking, and magnifying minor flaws, in an attempt to deny that he finds Mrs C very attractive indeed.