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Their visit did not continue long after the question and answer above mentioned; and while Mr. Darcy was attending them to their carriage Miss Bingley was venting her feelings in criticisms on Elizabeth's person, behaviour, and dress. But Georgiana would not join her. Her brother's recommendation was enough to ensure her favour; his judgement could not err. And he had spoken in such terms of Elizabeth as to leave Georgiana without the power of finding her otherwise than lovely and amiable. When Darcy returned to the saloon, Miss Bingley could not help repeating to him some part of what she had been saying to his sister. (45.12)
Georgiana seriously respects and looks up to her older brother, above anyone else. It's a good thing she has a solid role model/parent figure, unlike the Bennet girls.
"Not so hasty, if you please. I have by no means done. To all the objections I have already urged, I have still another to add. I am no stranger to the particulars of your youngest sister's infamous elopement. I know it all; that the young man's marrying her was a patched-up business, at the expence of your father and uncles. And is such a girl to be my nephew's sister? Is her husband, is the son of his late father's steward, to be his brother? Heaven and earth! – of what are you thinking? Are the shades of Pemberley to be thus polluted?" (56.63)
It seems like Lady Catherine has no problem with Elizabeth personally. Actually, Elizabeth was rather a favorite of Lady Catherine while staying with the Collinses. Rather, her primary objection to Elizabeth marrying Darcy is Elizabeth's family and their low connections. Lydia's scandalous relationship with Wickham is also a big problem.
Kitty, to her very material advantage, spent the chief of her time with her two elder sisters. In society so superior to what she had generally known, her improvement was great. She was not of so ungovernable a temper as Lydia; and, removed from the influence of Lydia's example, she became, by proper attention and management, less irritable, less ignorant, and less insipid. From the further disadvantage of Lydia's society she was of course carefully kept, and though Mrs. Wickham frequently invited her to come and stay with her, with the promise of balls and young men, her father would never consent to her going. (61.4)
Kitty hasn't had the benefit of very good parenting, and has always been really susceptible to Lydia's bad influence. However, Mr. Bennet seems to have wised up a bit (we notice he's not allowing Kitty to run loose at any balls). Also, now that she can escape to her older sisters' households, she's really improving.