Both Mary and Lucy are the heroines' all-too-successful rivals for their respective Mr Eds, and foist their company onto the heroines and amuse them superficially.

MP ch22> deriving no higher pleasure from her conversation than occasional amusement, and that often at the expense of her judgment

S+S ch22> Lucy was naturally clever; her remarks were often just and amusing; and as a companion for half an hour Elinor frequently found her agreeable; but ... her deficiency ... could not be concealed from Miss Dashwood, in spite of her constant endeavour to appear to advantage.