There was, of course, much that she did not know about Cecilia. But there would be time, for this tragedy was bound to bring them closer. (1.14.43)
Briony is thinking that jailing Robbie will bring her and Cecilia closer together. That would be a major, gigantic, unfixable error on her part, and the impetus for Briony beginning her novel.
Briony knew that if she had traveled two hundred miles to a strange house, bright questions and jokey asides, and being told in a hundred different ways that she was free to choose, would have oppressed her. It was not generally realized that what children mostly wanted was to be left alone. (1.1.13)
Briony is talking about her cousins, who have just arrived. They probably do want to be left alone to some degree, but at the same time they've also just left home because their parents are divorcing. So in that sense they probably wish that they hadn't been left so alone.
She had returned from Cambridge with a vague notion that her family was owed an uninterrupted stretch of her company. But her father remained in town, and her mother, when she wasn't nurturing her migraines, seemed distant, even unfriendly. (1.2.6)
Cecilia is being a teenager. She doesn't know what to do with herself, she can't get along with her parents, she's sneaking cigarettes every chance she gets. It would all be sort of funny if her family didn't ruin her life a couple hundred pages down the road…
Grace Turner was happy to take care of his laundry—how else, beyond hot meals, to show mother love when her only baby was twenty-three—but Robbie preferred to shine his own shoes. (1.8.17)
Grace Turner doesn't get a lot of screen time in the novel. As a parent, though, she comes off way better than Emily or Hermione or Jack.
Even being lied to constantly, though hardly like love, was sustained attention; he must care about her to fabricate so elaborately and over such a long stretch of time. His deceit was a form of tribute to the importance of their marriage. (1.12.6)
Emily is thinking about Jack's affair, and convincing herself that he cares about her because he's willing to construct elaborate lies about his infidelity for her. Who else in the novel shows they care by constructing elaborate fictions? Jack's youngest daughter, perhaps?
He paused to gather his courage. "It's a divorce!"
Pierrot and Lola froze. The word had never been used in front of the children, and never uttered by them. The soft consonants suggested an unthinkable obscenity, the sibilant ending whispered the family's shame. Jackson himself looked distraught as the word left him, but no wishing could bring it back now, and for all he could tell, saying it out loud was as great a crime as the act itself, whatever that was. (1.5.8)
Divorce would have been much less common in the 1930s, when this book is set. It's seen as an embarrassment, and as a result nobody has apparently explained it to the kids—not even (we learn a few sentences later) to fifteen-year-old Lola.
How could anyone presume to know the world through the eyes of an insect? Not everything had a cause, and pretending otherwise was an interference in the workings of the world that was futile, and could even lead to grief. Some things were simply so.
She did not wish to know why Jack spent so many consecutive nights in London. Or rather, she did not wish to be told. (1.12.7-8)
Emily thinks that imagination is dangerous and that it's bad for families. Briony proves her right, in some sense—she certainly messes up her family by "interfering in the workings of the world." At the same time, though, if Emily had had a little more imagination, and been willing to interfere a little more, maybe she could have prevented some of what happened?
Nearly every man here had a father who remembered northern France, or was buried in it. He wanted such a father, dead or alive. […] Rootless, therefore futile. He wanted a father, and for the same reason, he wanted to be a father. It was common enough, to see so much death and want a child. Common, therefore human, and he wanted it all the more. When the wounded were screaming, you dreamed of sharing a little house somewhere, of an ordinary life, a family line, connection. (2.244)
Robbie at war imagines a stable family life. There's something ironic about this though, since the Tallises' ordinary family life basically destroyed him. Still, you can hardly blame the guy for wanting to get out of France. And the Tallis family house, for all its problems, certainly looks like an appealing place to be from the middle of a war.
"They turned on you, all of them, even my father. When they wrecked your life they wrecked mine. They chose to believe the evidence of a silly, hysterical little girl. In fact, they encouraged her by giving her no room to turn back. She was a young thirteen, I know, but I never want to speak to her again. As for the rest of them, I can never forgive what they did. Now that I've broken away, I'm beginning to understand the snobbery that lay behind their stupidity. My mother never forgave you your first. My father preferred to lose himself in his work. Leon turned out to be a grinning, spineless idiot who went along with everyone else." (2.91)
Cecilia's view of Leon here is very different than her view earlier in the novel (for an example see 1.9.66). His easy-going, see-the-best-of-everyone attitude turns out to look a whole lot of spinelessness in the face of crisis. And everyone else comes off horribly as well. This is the one-paragraph case for the prosecution against the Tallis family. It seems fairly damning.
How delightful to be at the heart of such a good-willed reunion. […] We ranged in age from three months to [Leon's] eighty-nine years. And what a din of voices, from gruff to shrill, as the waiters came round with more champagne and lemonade. The aging children of distant cousins greeted me like long-lost friends. (4.34)
The end of the novel has the happy, cheerful family scenes that the beginning mostly lacks. The family here is a cheerful, bustling affair, rather than a big clump of resentment and jealousy and frustration. Maybe the younger generation of Tallises is just happier, or maybe families look less tense when you're looking at them from the perspective of a seventy-seven-year-old great-aunt than when you look at them from the perspective of a thirteen-year-old.