I think Flora is younger because of how Jo treats her vs. the MC: one is showered with praises, another is demanded more responsibility. And at the start it seems the MC was the one cooking breakfast for Flora, that's typically you'd see the older sibling doing to the younger.
The school works with some weird system. Every student shown is in the same classes; except perhaps the cheerleaders, I do expect them to be on the same year as Kate.
That said I think canonically it's simply unspecified which is younger. Flora could be younger, older, or both could be dizygotic twins (more on that below).
I was under the impression that she's a twin. That's why they're in the same grade. Also drives home the point that MC went kinda crazy when Flora left him. She's the twin that he thought would always be there with him no matter what.
MC and Flora are also fairly similar in appearance based on what we've seen. Freckles n all.
There are two types of twins:
- monozygotic (one egg) - the egg for some reason splits in the womb, and two children are born. Their genetic composition is 100% identical, and they look confusingly identical.
- dizygotic (two eggs) - the mother's ovary released two eggs at the same time, and both got fertilised. Their genetic composition is ~50% identical, just like any other pair of siblings.
If Flora and the MC are twins, they must be dizygotic - otherwise they'd have the same sex. So if they look similar it's simply because they're siblings, being or not twins should have no impact on that.
