How we cite our quotes: (Chapter.Paragraph)
Quote #1
[Cecilia] sat on a barstool, staring into her punch glass, and the shapeless bag of a dress fell over her. She had colored her lips with a red crayon, which gave her face a deranged harlot look, but she acted as though no one were there.
We knew to stay away from her. (1.41-42)
Even in the middle of a party, Cecilia finds herself isolated from the rest of the group. She's different, and it shows in her posture, her strange makeup, and her behavior. She gives off a vibe that people should keep away, and they do. She's feeling isolated even in the middle of a party. Her sense of feeling different isolates her.
Quote #2
As the diary progresses, Cecilia begins to recede from her sisters and, in fact, from personal narrative of any kind. The first person singular ceases almost entirely, the effect akin to a camera's pulling away from the characters at the end of a movie, to show, in a series of dissolves, their house, street, city, country, and finally planet, which not only dwarfs but obliterates them. (2.18)
In her own writing, Cecilia seems to be isolated from herself. She loses her sense of identity as she loses her narrative "I". She seems to want to fade out of the world, where she already feels isolated. Definitely a warning sign of depression. Psychologists call this "decathexis": taking all the emotional energy you usually invest in the world and other people and pulling it back. You just don't care anymore.
Quote #3
We never learned whether Mrs. Lisbon caught Lux as she tried to sneak back inside, but for whatever reason, when Trip tried to make another date to come sit on the couch, Lux told him she was grounded, and that her mother had forbidden any future visits. (3.59)
After Trip visits Lux at her home, she sneaks into his car and they have a wild sexual encounter. Even if Mrs. Lisbon doesn't catch Lux in the act, she seems to have a sixth-sense about it and attempts to keep Lux from more of the same by isolating the girls, refusing all visitors. Lux's behavior just served to validate Mrs. Lisbon's fears about the world outside the family. She sees isolation as safety.