You've brought up something that crossed my mind while reading this section.

Mrs. Dashwood and Marianne seem to be two of a kind, as has been discussed in several previous threads. They seem to be much closer than Mrs. Dashwood and Elinor, even though Mrs. Dashwood relies upon Elinor's consistency and reason.

So, why would Mrs. Dashwood NOT inquire of Marianne about the status of her relationship with Willoughby? Would it have violated a social norm of the time for a parent to inquire? Marianne is only sixteen. What parent would not be concerned at the rapid development of what appears to be a serious relationship? Is Mrs. Dashwood shrugging her responsibility as a parent? Or was it the father's domain in the Regency period to monitor the intentions of a daughter's suitors? Maybe Mrs. Dashwood has no idea of how to handle the responsibility in her husband's place?