One important theme in all of JA novels is the relationship between sisters. Here in P&P, and in these particular chapters we see two different pair of sisters -- in the same family.

Her tour to the Lakes was now the object of her happiest thoughts; ... and could she have included Jane in the scheme, every part of it would have been perfect.

"But it is fortunate," thought she, "that I have something to wish for. Were the whole arrangement complete, my disappointment would be certain. But here, by carrying with me one ceaseless source of regret in my sister's absence, I may reasonably hope to have all my expectations of pleasure realized. A scheme of which every part promises delight can never be successful; and general disappointment is only warded off by the defence of some little peculiar vexation." (Ch. 42)

Elizabeth is a bit sad that Jane could not accompany her and the Gardiners. She consoles herself thinking that if Jane went, everything would be perfect and maybe something bad could happen. We are not told about Jane thoughts, but I suppose that she was happy that Elizabeth made this trip.

Contrast this with Kitty and Lydia:

The rapture of Lydia on this occasion, her adoration of Mrs. Forster, the delight of Mrs. Bennet, and the mortification of Kitty, are scarcely to be described. Wholly inattentive to her sister's feelings, Lydia flew about the house in restless ecstacy, calling for every one's congratulations, and laughing and talking with more violence than ever; whilst the luckless Kitty continued in the parlour repining at her fate in terms as unreasonable as her accent was peevish. (Ch. 41)