beyond the mere social conventions and awkwardness. Remember that she decided, on Elinor's advice, to "for their sakes avoid a total breach with their brother." (Ch. 1)

Worst case scenario, something happens to Mrs. Dashwood before any of the girls are well married - John Dashwood is the last hope they have of someone to take care of them. Coming back into the John and Fanny house as a poor relation is a bad alternative, but better than the others available (governessing? I don't know what else might even possibly become of them.) If she confronts John face to face and it turns ugly, or she forces him to embarrass himself by admitting out loud that he has no intentions of helping, she might take even that bad alternative away from her girls.