Charmed Life, by Dianne Wynne Jones
Kids' fantasy novel. This was fun, from a friends' favourite-childhood-author.---
Tangential thoughts: I tend to be skeptical about any categorical things that people use to explain their personality. The obvious punchbag here is astrology, or used to be, but nowadays seemingly 50% of my peers semi-seriously believe in astrology and I really don't know what to do with that information.* But I'm skeptical of lots of other category-induced personality claims as well.
Anyway, birth order effects are one of the few things that I could actually believe consistently affect your personality etc -- I feel like I spent my whole childhood trying to keep up with my slightly older brother, who was always smarter and better than me, and you can sort of imagine that a similar dynamic is somewhat-consistently true of lots of other people too, such that Second Children show some kind of somewhat-consistent personality patterns.**
At the same time, this is exactly the sort of just-so story about the self that I'm so deeply skeptical of. In the same situation but with a slightly different personality, I could easily imagine that I could have become utterly dejected by always being worse than my older brother, and given up on school completely; or I could have gotten really into being good at some Other Thing, some way to compete without competing; and billions of younger-siblings out there have presumably gone down many of these different routes.
I don't want to give any spoilers but this discussion is not entirely irrelevant to Charmed Life.
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Unrelated fact: my dear friend who writes children's books told me that children's books generally-always have to start by getting the parents out the way (so that the child can have autonomy and protagonism, I think?), and that often this just means killing the parents suddenly and unceremoniously at the very start of the book. I hadn't really noticed this as a kid but... it's kind of wild, no?, that all of children's literature is a bunch of orphans, that parents just die all over the place in fiction and both the child-readers and their guardians are totally fine with this?
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One thing that's really great about Charmed Life is how well it captures the mentality and attitudes and interests of a child -- the psychology of a child, especially the psychology of a child with a sibling. And to the extent that one of the "uses" of fiction is to allow us to experience other possible minds.... maybe reading children's books is actually an unusually useful, broadening experience? Children are aliens in a very meaningful way, and their minds are both more-different from my own (I think) than most adult minds are, but also of course a type of mind that I once inhabited, so should presumably be able to access if I just get properly reminded of it? (This is presumably less valuable for people who are already more in touch with their childhood selves, but still).
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* ok, really, does anyone know how and when astrology became popular in certain demographics again? My first memory of it was around 2015 I think, I went to a party and a girl I'd just met said "what star sign are you? Gemini? You're going to be alone forever," it was very strange and memorable
** which makes you think that maybe the ideal way to raise a child if you want them to learn fast is with a series of "pacers", children slightly-older than themselves who they can "chase" and then catch up to and then be replaced with a new aspirationally-smarter child. But I don't know if the motivation to keep up would be nearly as strong if the older child wasn't this one friend/rival who you'd known since birth, and also obviously it does not seem (ahem) always healthy to maximise children's learning speed over their wellbeing