Jack hasn’t told Gwendolen about his excessively pretty young ward, Cecily, because he fears that Gwendolen might be jealous.
In The Importance of Being Earnest, Algernon questions Jack about why he has never mentioned Cecily, and Jack responds:
"My dear fellow, there is nothing improbable about my explanation at all. In fact, it’s perfectly ordinary. Cecily and Gwendolen are perfectly certain to be extremely great friends. I’ll bet you anything you like that half an hour after they have met, they will be calling each other sister."
This statement is ironic because, when Cecily and Gwendolen do meet, they immediately become rivals, mistakenly believing they are both engaged to "Ernest" (Jack’s fictional alter ego).
Jack’s omission highlights the themes of deception and mistaken identity in the play, as well as Wilde’s satire of Victorian courtship and romantic ideals.
