She's a third-year middle-schooler, one year younger than him. Chill.
Especially since the osananajimi girl was already there just a few chapters back.
"Onii-san" is also used as a casual form of address for older guys a girl may be close to, especially if they've known each other for a long time. It's far from impossible for kareshi to also be onii.
Oh, I know it did. But the story takes a few different turns away from the WN at the end of the first LN, so i doubt that's gonna impact the adaptation.