But here it’s not a reveal, it’s given as initial exposition. The companion trope to the above is that the young person is not simply disgusted with the mom on moral or aesthetic grounds, but that one or more of the transient “boyfriends/uncles, etc.” has at least tried something with the young person.
That doesn’t mean it’s an inevitable development, but it’s certainly a possible move on the board.
So far there's no obvious reason to assume such though; and Chika's home environment is quite amply screwed up enough to deeply and negatively affect someone up to and including for life as is without recourse to such lurid dramatic devices.
Laying the basic situation bare to the audience right in the opening strikes me as a very deliberate narrative choice - instead of being left in as much of the dark as a single specific viewpoint character and given the reveal "in real time" as they (more or less gradually) learn about it in-universe, we're instead told it right out of that gate from the PoV of the character in question. Or to put it differently and more concisely, the goal is not the audience uncovering a "mystery" alongside the cast but rather watching the cast discover things - in this case about each other - in-universe that the audience is already privy to.
Contrariwise if you took out the frequent flips to Chika's PoV we'd have a fairly classic (if not outright cliché) setup of Usa trying to figure out why her totally-not-crush-honest is such a tightly-wound unsociable curmudgeon.
