In Chapter 50 Mr Gardiner mentions (in his follow-up letter to Mr Bennet) that Lydia wants to stop by Longbourn with Wickham before they travel north to join Wickham's new regiment. Mr Bennet does not want to welcome them home at all.

"But Jane and Elizabeth, who agreed in wishing, for the sake of their sister's feelings and consequence, that she should be noticed on her marriage by her parents, urged him so earnestly, yet so rationally and so mildly, to receive her and her husband at Longbourn, as soon as they were married, that he was prevailed on to think as they thought, and act as they wished."

Jane & Elizabeth are considerate of Lydia, though we might all agree that she is hardly deserving of any such consideration. They are kind sisters in this regard.

Then in Chapter 51 we read

"Their sister's wedding day arrived; and Jane and Elizabeth felt for her, probably more than she felt for herself. ... . Their arrival was dreaded by the elder Miss Bennets, and Jane more especially, who gave Lydia the feelings which would have attended herself, had she been the culprit, and was wretched in the thought of what her sister must endure."

This is further evidence, if we needed it, that Jane & Elizabeth are both good people -- women with moral standards who feel bad about misbehaviour and worse. Jane, especially, assumes that Lydia would have the feelings that she (Jane) might have and is totally empathetic with her youngest sister on that account.

In the end, however, both Elizabeth & Jane realize that they have been empathetic and sympathetic without cause.

"The easy assurance of the young couple, ... . Elizabeth was disgusted, and even Miss Bennet was shocked. Lydia was Lydia still -- untamed, unabashed, wild, noisy, and fearless."

So our Lydia has not changed a bit. She may be, in fact, even bolder due to her sudden rise in importance --

"Ah, Jane, I take your place now, and you must go lower, because I am a married woman!"