The idea of romantic delicacy as discussed in Ch. 16 seems a little foreign to my modern mind .

"Why do you not ask Marianne at once," said she, "whether she is or is not engaged to Willoughby? From you, her mother, and so kind, so indulgent a mother, the question could not give offence. ... She used to be all unreserve, and to you more especially."

"I would not ask such a question for the world. Supposing it possible that they are not engaged, what distress would not such an inquiry inflict! At any rate it would be most ungenerous. I should never deserve her confidence again, after forcing from her a confession of what is meant at present to be unacknowledged to any one. I... I would not attempt to force the confidence of any one, of a child much less, because a sense of duty would prevent the denial..."

Elinor thought this generosity overstrained, considering her sister's youth, and urged the matter farther, but in vain; common sense, common care, common prudence, were all sunk in Mrs. Dashwood's romantic delicacy.

It's interesting that Mrs. Dashwood said "supposing they're not engaged", when she had gone on and on about how they must be engaged right after Willoughby left.

Elinor seems to have some of her mother's same romantic delicacy, because why didn't she just gently ask Marianne herself? She seems to think it would be better to come from her mother, so she urges her to do it. Once her mother says no, it would not be right for Elinor to do it herself. But it seems natural to me that you should be able to talk over such matters with your close family.

It seems Marianne herself has no romantic delicacy, because she had no compunction in asking Elinor if she was engaged in ch. 4.

"When you tell me to love him as a brother, I shall no more see imperfection in his face, than I now do in his heart.