...even Darcy admitted in his letter that there was nothing wrong with Jane personally, and that he was (probably) mistaken about her not being in love with Bingley. Elizabeth clearly still feels that Jane and Bingley getting engaged is exactly the right thing for both of them:

Elizabeth, who was left by herself, now smiled at the rapidity and ease with which an affair was finally settled, that had given them so many previous months of suspense and vexation.

"And this," said she, "is the end of all [Bingley's] friend's anxious circumspection! of all his sister's falsehood and contrivance! -- the happiest, wisest, most reasonable end!"

...in spite of [Bingley] being a lover, Elizabeth really believed all his expectations of felicity to be rationally founded, because they had for basis the excellent understanding and superexcellent disposition of Jane, and a general similarity of feeling and taste between her and himself. (ch.55)

Now, I admit she doesn't actually accept Darcy until after the Jane/Bingley matter is resolved (since he doesn't ask her), but I do still feel her sisterly loyalty is a touch lacking here. :-(