I mean the question pretty much says it all.
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She is concerned that Mrs. Smith calls Willoughby away because she doesn't approve of his engagement to Marianne, and that possibly he'll never return because of it. The section below supports the answer;
"I am persuaded that Mrs. Smith suspects his regard for Marianne, disapproves of it, (perhaps because she has other views for him,) and on that account is eager to get him away;—and that he business which she sends him off to transact is invented as an excuse to dismiss him.
This is what I believe to have happened. He is, moreover, aware that she does disapprove the connection, he dares not therefore at present confess to her his engagement with Marianne, and he feels himself obliged, from his dependent situation, to give into her schemes, and absent himself from Devonshire for a while. You will tell me, I know, that this may or may not have happened; but I will listen to no cavil, unless you can point out any other method of understanding the affair as satisfactory at this. And now, Elinor, what have you to say?”
Sense and Sensibiltiy