the Musgroves ever knew. Remember from Ch. 4

She was assisted, however, by that perfect indifference and apparent unconsciousness, among the only three of her own friends in the secret of the past, which seemed almost to deny any recollection of it. She could do justice to the superiority of Lady Russell's motives in this, over those of her father and Elizabeth; she could honour all the better feelings of her calmness; but the general air of oblivion among them was highly important from whatever it sprung;

The three being LR, SW and Elizabeth. No one talked about it.
Later, in Ch. 6
especially, as she had satisfied herself in the very first half-minute, in the instant even of introduction, that there was not the smallest symptom of any knowledge or suspicion on Mrs. Croft's side to give a bias of any sort. She was quite easy on that head, and consequently full of strength and courage