The narrative seems to frame Cindy as being essentially very bratty, when she's actually got a legitimate reason to be annoyed. It's one thing to not like the woman her father is seeing. It's another for him to announce that he's gotten her pregnant and that she and her two daughters will also have to move into their house. Cindy is actually remarkably mature about the whole thing; while she doesn't pretend to be happy, she just grits her teeth and says they'll all have to make the best of it.
It's easy to see Aishling as a massive hypocrite. She has No Sympathy for Cindy at all. In spite of the girl's abrasive temper, she's lost her mother and had her father dating one of her teachers within a few months. Aishling does not go to that school, both her parents are alive and her life is barely affected by Margaret's initial romance with Philip. She actually treats Bob horribly, for the same reasons she criticises Cindy for. Then for some reason, she's horrified when Bob started dating Cindy even if she (Aishling) is the one who dumped him twice and treated him horribly. To some readers it comes off as Laser-Guided Karma.
As noted under Values Dissonance below, Margaret's reason for marrying Richard - fear that Philip might take legal action against her - is pretty flimsy. One must wonder if she's using it as an excuse. Or perhaps she's worried about the girls at her school seeing her as a hypocrite.
Angst? What Angst?: Cindy has this reaction to her father after her mother's funeral. She feels as though they should keep the shutters closed and is annoyed when he doesn't seem to be more sad. She eventually learns he's putting on a brave face.
Fridge Horror: It's mentioned that Philip pays for Aishling and Alva's school. Thus giving him a very easy target to attack Margaret with if things aren't going his way.
Cindy and Aishling bonding at one point. Cindy comes home from drinking with Imelda with just one shoe, and it's said that Aishling lets her into the house and the two have a good giggle about it. It does suggest that there is potential for the two to become friends.
Bob appears to be a genuine Nice Guy. Despite the nasty way Aishling treated him, he immediately comes to her aid when she thinks Alva might be pregnant.
Cindy. Let's face it - you're fifteen and your mother has just died of an illness and your father marries one of your teachers in less than a year because he's gotten her pregnant. Then they all have to move in together. Her temper is a little understandable.
Alva, as annoying as she can be, she's also a young girl who still misses her father and apparently cries every night.
Unintentionally Sympathetic: Aishling has very little sympathy for Cindy in her diary. The narrative also seems to view her as a brat who needs to mature. This is a girl that first has had to deal with her mother suffering from a terminal illness, dying from said illness, her father remarrying and three new people moving into her house all in the space of a year. More superficially she also had to do her Junior Cert - which is quite a stressful experience for a fifteen-year-old.
Any non-Irish readers (or younger Irish readers) would probably be a little shocked at some of the casual racism displayed in the book. Cindy's mother was American and Cindy frequently recalls her father trying to get her to stop pronouncing her words with an American accent. He also insisted her name not be spelled 'Cyndi' because it was "too American". For many years, it was expected that anyone from a foreign country who migrated to Ireland was expected to lose their accent and start using Irish slang words and pronunciations. Nowadays, with a larger rate of actual immigrants in Ireland, this sentiment has largely died out.
Cindy also mentions her friends going out drinking after getting their exam results - her friends would be fifteen at the time which would be a surprise to anyone not familiar with Irish drinking culture. Cindy herself also gladly has plenty of wine at meals without her father objecting. Alva is fourteen and has also had champagne a few times. While this would have been illegal at the time, this would be completely socially unacceptable in Ireland today.
Margaret marries Richard partly because she's afraid that a conservative judge will rule against an unmarried woman raising two daughters. This is Ireland before divorce was even legal. In the 2000s such an attitude would be gone completely, and a court would be more likely to rule in favour of the mother than the father - especially with Philip's alcoholism to think of.
The Woobie: Margaret has to put up with a really awkward situation. She's a single mother supporting two teenage daughters, while also clashing with her ex-husband. Part of the reason she marries Richard is because she's worried that her ex might take legal action to have Alva taken off her.