Also, aware that she is unimportant- an impediment, even- to her mother, it is not unlikely that Cynthia looked for affection and validation from others, so made herself as pleasing company as possible. She freely admits to Molly that she doesn't just want people to like her, she needs them to. In addition, between school in Paris and the fine houses where she spends her vacations, Cynthia has acquired a taste for a lifestyle which, as a member of the poor gentry, she would normally be disqualified from. What better way to assure she's still invited to the best and most glittering social events than by making herself agreeable, indispensable company? But her fascination remains a surface one- there is little underneath.
Likewise, with regard to the Preston problem, she talks Molly into meeting with him and dealing with his anger and bitterness. This is manifestly unfair; Cynthia is responsible for the situation, and should have to deal with the unpleasantness involved with settling the matter. It is easier, however, to avoid this by imposing on Molly, and she does so, not even thinking of the possible consequences. While she traipses off to London, unconcerned and unaware, Molly's reputation is almost ruined. Cynthia never has to deal with the fallout from her actions, and doesn't seem to feel that she ought to. When Dr Gibson finally takes her to task for her actions, she plays the victim, crying and saying that everyone has been cruel to her- childish behaviour. Also, like a child, when in trouble she generally threatens to run away, saying that she's going to Russia to become a governess. She doesn't really mean it of course... it's just another way of bringing other people around to deal with her problems so that she doesn't have to.
Also, while Cynthia frankly admires the integrity of people like Molly, Roger, and Dr. Gibson, she never attempts to emulate it. She freely and cheerfully admits her own faults and character defects, but seems to feel that her frankness should excuse her from actually having to do anything to correct these flaws. What use is it to be self-aware if you never strive to improve yourself?