There was more than one Lady Duff Gordon. The one you mentioned above also wrote Letters from Egypt. Here is a short bio- what a coincidence both were named Lucie (Lucy).
Lucie Duff Gordon was a contemporary and friend of Dickens, Carlyle, Tennyson, Thackeray, and all the intellectual luminaries of her day. She was known as one of London's most charming hostesses. From life at the center of mid-19th century English high society, Lucie Duff Gordon was exiled by bad health to Egypt, a land so foreign that most Europeans were glad to go home after a quick dash up the Nile. Instead, Lucie found a new life as the queen of Upper Egypt, much loved by the fellahin whose language, customs, and religion she learned and admired over her seven years there. Her Letters From Egypt became a best seller and brought her great fame." (Copyright (c) Brian Hunt, 2004)
Yes, Lucie Duff Gordon (1821-1869) was Cosmo Duff Gordon’s aunt by marriage, being the wife of his father’s elder brother. "Lucile" does tell in her memoir a funny story of how, when she met Teddy Roosevelt in 1908, the President mistook her for the other, earlier Lady Duff Gordon.
I know you are interested in the Duff Gordons, so you might like to read some info I posted to ET some time ago about their descendants. Here’s a link:
Lucie Duff Gordon, the elder, has had a wonderfully appreciative account of her life published not long ago. It’s called "A Passage to Egypt" by Katherine Frank. It’s superbly written and is even more emotional for the author’s own connection to the story. It’s a must read for anyone interested in Victoriana and travel history.