How we cite our quotes:
"The harm of it is, that her father [Edgar] would hate me if he found I suffered [Cathy] to enter your house; and I am convinced you have a bad design in encouraging her to do so." (21.36)
Nelly clues in to Heathcliff's ill intent, but that doesn't stop her from being suckered into going back to the Heights.
Mr. Linton was alarmed and distressed, more than he would acknowledge to me. In the morning, Catherine learnt my betrayal of her confidence, and she learnt also that her secret visits were to end. (24.84)
It is never entirely clear where Nelly's loyalties and intentions reside. Is she afraid to say no to Catherine? Does she actually enjoy the drama up at the Heights? Think about it!
And then I remembered Mr. Edgar's stern rebuke of my carrying tales; and I tried to smooth away all disquietude on the subject, by affirming, with frequent iteration, that that betrayal of trust, if it merited so harsh an appellation, should be the last. (14.47)
Nelly tries to abide by her master's dictates, but her sympathy to lovers is greater than perhaps even she will admit. Again, why would she continue to go to the Heights and introduce Heathcliff to young Catherine?