Quote 1
"Perhaps, Miss Marianne," cried Lucy, eager to take some revenge on her, "you think young men never stand upon engagements, if they have no mind to keep them, little as well as great." (35.27)
Ooh, ouch. That's a low blow on Lucy's part – she's starting to take her jealousy out on Marianne as well as Elinor, with this personal jab at Marianne's breakup with Willoughby.
Quote 2
Lucy went on. "I am rather of a jealous temper, too, by nature, and from our different situations in life, from his being so much more in the world than me, and our continual separation, I was enough inclined for suspicion, to have found out the truth in an instant, if there had been the slightest alteration in his behaviour to me when we met, or any lowness of spirits that I could not account for, or if he had talked more of one lady than another, or seemed in any respect less happy at Longstaple than he used to be. I do not mean to say that I am particularly observant or quick-sighted in general, but in such a case I am sure I could not be deceived." (24.6)
Lucy issues a clear warning to Elinor here – back off, sister!
Quote 3
"Good heavens!" cried Elinor, "what do you mean? Are you acquainted with Mr. Robert Ferrars? Can you be -- -- ?" And she did not feel much delighted with the idea of such a sister-in-law.
"No;" replied Lucy, "not to Mr. Robert Ferrars -- I never saw him in my life; but," fixing her eyes upon Elinor, "to his elder brother."
What felt Elinor at that moment? Astonishment, that would have been as painful as it was strong, had not an immediate disbelief of the assertion attended it. She turned towards Lucy in silent amazement, unable to divine the reason or object of such a declaration; and though her complexion varied, she stood firm in incredulity, and felt in no danger of an hysterical fit, or a swoon. (22.7)
This is totally a veiled declaration of war – and the thing is, the conflict between Elinor and Lucy has to play out in outwardly civil forms! Lucy knows that Elinor can't strike back at her, since she has nothing to go on; Elinor herself is at a total loss for what to do in the face of this declaration.