Old Friends and New Fancies is set in the same time as Austen's own novels and is similarly structured, with a focus on the challenges of matchmaking among pairs of lovers kept apart by various social and economic tensions. It has something of a postmodern overtone in that it mixes together characters from all six of Austen's major novels, creating an enormously extended network of friends, relations, and acquaintances. For example, Elizabeth (Bennet) Darcy (of Pride and Prejudice), Elinor (Dashwood) Ferrars (of Sense and Sensibility), and Anne (Elliot) Wentworth (of Persuasion) are all friends. Despite the fact that Brinton provides a full list of characters (sorted by their source books), keeping the cast straight is something many readers complain about, since quite a few of the characters are only mentioned in passing. Most of the characters are recognizably the same, though several have improvedâamong them Kitty Bennet and Tom Bertramâwhile George Knightley is somewhat sourer and Mary Crawford much less lively than in Austen's depictions. The largest single change from Austen's own books is that Marianne Dashwood's husband, Colonel Brandon, has died before the book opens. Although the book jacket proclaims that Brinton mixes in "new characters of the author's devising," none of the new characters are of any great importance. Because many of the key characters hail from Pride and Prejudice, some critics treat Old Friends and New Fancies primarily as a sequel to that particular book. Mansfield Park is the next best represented novel in terms of major characters.
The chief protagonists of Old Friends and New Fancies are three young women, all unmarried at the outset: Georgiana Darcy and Kitty Bennet (Pride and Prejudice) and Mary Crawford (Mansfield Park). The novel begins some six months after the marriage of Elizabeth Bennet and Fitzwilliam Darcy; Darcy's sister Georgiana and cousin Colonel Fitzwilliam have gotten engaged but are not very happy together, and they soon break it off. Col. Fitzwilliam goes to visit Lady Catherine de Bourgh in Bath with the Darcys, where he meets and falls in love with Mary Crawford. Robert and Lucy (Steele) Ferrars have been cultivating Lady Catherine, and Lucy is hoping that Col. Fitzwilliam will marry her sister Anne. The Ferrars take the opportunity to slander Mary to Lady Catherine, resulting in her banishment from Lady Catherine's circle. Lonely and defiant, Mary begins spending time with an admiring Sir Walter Elliott, leading to a rumor that they are about to marry. Hearing this and feeling that his comparative poverty and lack of title make him a poor match for Mary, Col. Fitzwilliam removes himself from the scene, going to Ireland for a time. They are only reconciled after Col. Fitzwilliam is badly injured in a fall from his horse while hunting.
Meanwhile, Kitty Bennet has gone to London as a protegĂŠ of Emma (Woodhouse) Knightley. Although less flighty than formerly, she falls madly in love with William Price, a friend of the Knightleys and a naval officer who is the younger brother of Fanny (Price) Bertram of Mansfield Park. Georgiana visits Kitty in London, where she is introduced to William at a ball given by the Knightleys. Kitty later goes to stay with Elizabeth and Georgiana at Pemberley, and they try unsuccessfully to rein in her expectation of receiving a proposal of marriage from William. The Darcys give a ball at which William, instead of proposing to Kitty, declares his love to Georgiana. Georgiana, out of consideration for Kitty's feelings and confusion about her own, initially rebuffs William, but eventually the two become engaged, while a sobered Kitty pairs up with clergyman James Morland, whom Darcy has installed in a local parish. The latter denouement was hinted at by Austen herself in her letters, where she mentions that she can imagine Kitty married to a Derbyshire clergyman. Almost as an afterthought, Brinton also pairs up Tom Bertram and Isabella Thorpe, completing a sequence in which all of Brinton's characters destined for matrimony become engaged to someone from a different Jane Austen novel.

Question: Why do Robert and Lucy talk bad about Mary?
Answer: Because Lucy wants the Colonel to marry her sister.
Alternative answer: to break-up Col. Fitzwilliam and Mary. so he will marry Lucy's sister