Marianne was silent; it was impossible for her to say what she did not feel, however trivial the occasion; and upon Elinor, therefore, the whole task of telling lies when politeness required it, always fell.

Ahhhh, feelings: Marianne's moral compass. Since she does not like Lady Middleton, she cannot make herself say anything pleasant about her. To do so would satisfy the demands of society at the expense of her delicate conscience.

]Marianne, on the other hand, does actually lie in company. Back in the first week's reading, when Mrs. Jennings was trying to find out who Elinor liked, Marianne says to Margaret, "you know that all this is an invention of your own, and that there is no such person in existence." Is this not a lie for the sake of
propriety?