If Willoughby and Marianne are engaged, wouldn't he have to ask Mrs. Dashwood, as she is under age? Even if he cannot marry her at present?

As Elinor tells her mother:

He had just parted from my sister, had seen her leave him in the greatest affliction; and if he felt obliged, from a fear of offending Mrs. Smith, to resist the temptation of returning here soon, and yet aware that by declining your invitation, ... , he should seem to act an ungenerous, a suspicious, part by our family, he might well be embarrassed and disturbed. In such a case, a plain and open avowal of his difficulties would have been more to his honour, I think...."

Elinor is saying that Willoughby should tell Mrs. Dashwood: "I love Marianne, and wish to marry her, but my cousin Mrs. Smith has other ideas; we must wait for some time, etc."

Mrs. Dashwood's answer is curious:

"Had he been in a situation to act independently and marry immediately, it might have been odd that he should leave us without acknowledging everything to me at once: but this is not the case. It is an engagement in some respects not prosperously begun, for their marriage must be at a very uncertain distance; and even secrecy, as far as it can be observed, may now be very advisable."

Previously, Elinor has said that she understood that they kept the engagement from Mrs. Smith, but why from her family? Her mother says that she doesn't need to be told. (Really?)