Yes, they’re so much alike they could be sisters – and now they will be. You might want to check out "Gwen and Cecily" in Gwendolen's "Character Analysis" for more information on what the two women have in common.
Part of what makes Cecily attractive to Algernon is her seeming simplicity. She’s not intellectual like Gwendolen, who very early on scolds Jack, "Ah! that is clearly a metaphysical speculation, and like most metaphysical speculations has very little reference at all to the actual facts of real life, as we know them" (I.149). We can’t really imagine Cecily talking about metaphysics – or facts, for that matter. Cecily does everything she can to vigorously avoid Miss Prism’s attempts to educate her. She’s innocent – Gwendolen might say ignorant. She waters the plants, writes in her diary, and waits for the day that Ernest will come and propose.
Cecily may hate German diction, but she loves stories. She gets so excited when Miss Prism reveals that she has written a three-volume novel. And Cecily describes Algernon’s desires to reform himself as "Quixotic," indicating that she’s read the novel by Cervantes in which a man with delusions of grandeur has numerous adventures. Like Algernon, Cecily loves a good bit of fiction – and her favorite writer is herself.
In her diary, she makes long entries recording romantic events that are entirely fictional. We love this one:
Worn out by your entire ignorance of my existence, I determined to end the matter one way or the other, and after a long struggle with myself I accepted you under this dear old tree here. The next day I bought this little ring in your name, and this is the little bangle with the true lover's knot I promised you always to wear.
Did I give you this? It's very pretty, isn't it? (II.215-216)
The comedy is in all the intricate details – the tree, the ring, the bangle with the true lover’s knot – and the fact that Algernon doesn’t quietly send for a straightjacket. In the fantastical comic world of The Importance of Being Earnest, Algy just rolls with it. Interestingly enough, Algernon takes Cecily's eccentric behavior as yet another sign that she is the girl for him.
Cecily may be younger, less fashionable, and less sophisticated than Gwendolen, but she can give as good as she gets. Check this out:
Are there many interesting walks in the vicinity, Miss Cardew?
Oh! yes! a great many. From the top of one of the hills quite close one can see five counties.
Five counties! I don't think I should like that; I hate crowds.
[Sweetly] I suppose that is why you live in town? [Gwendolen bites her lip, and beats her foot nervously with her parasol.]
[Looking round] Quite a well-kept garden this is, Miss Cardew.
So glad you like it, Miss Fairfax.
I had no idea there were any flowers in the country.
Oh, flowers are as common here, Miss Fairfax, as people are in London. (II.299-306)
Gwen got schooled. The girls are superficially civil here because the butler, Merriman, is setting up the tea. Neither Gwendolen nor Cecily can commence the scratching and hair-pulling in front of him. But Cecily proves that, though she may have been raised in the country, she’s primed to enter London society as Algernon’s wife. She’s quick-witted and determined, and with the guidance of her new sister-in-law, Gwendolen, she’ll be formidable. In time, we can almost see her taking on Lady Bracknell.