In ch.37, Lady Catherine holds forth on how necessary it is for young ladies to have someone to look after them on their travels, and describes how well guarded Georgiana was last summer when she went to Ramsgate:
"Young women should always be properly guarded and attended, according to their situation in life. When my niece Georgiana went to Ramsgate last summer, I made a point of her having two men-servants go with her. Miss Darcy, the daughter of Mr. Darcy, of Pemberley, and Lady Anne, could not have appeared with propriety in a different manner."
I suspect that Lady Catherine is among those whom Darcy has kept in the dark about Georgiana's intended elopement - not that difficult to do, actually, since practically speaking the elopement never took place. So, Elizabeth is now better informed about a Darcy family secret than one of Georgiana's closest relatives...