I wouldn't worry about being too nasty to Lucy; there have already been several posts about the way Austen delineates her character and she is clearly intended to be unsympathetic. In connection with this situation, I always think of this passage from Charlotte Bronte's Jane Eyre:

"If Miss Ingram had been a good and noble woman, endowed with force, fervour, kindness, sense, I should have had one vital struggle with two tigers--jealousy and despair: then, my heart torn out and devoured, I should have admired her--acknowledged her excellence, and been quiet for the rest of my days: and the more absolute her superiority, the deeper would have been my admiration--the more truly tranquil my quiescence."

I think Elinor finds herself in much the same position: her pain arises not only from the fact that Edward is spoken for, but spoken for by such a woman as this. More from the same chapter: "She advocated a high tone of sentiment; but she did not know the sensations of sympathy and pity; tenderness and truth were not in her."