Not really, flirting would be something quite more coquettish. They're children, though, and flirting actually requires quite some social knowledge and experience, which they obviously shouldn't have. Though Yamada could have, considering she's been exposed to the adult world for quite a while. MC, though, nah.
...I actually have a hard time recalling any shounen romcom or romance where there is flirting. Classics like Maison Ikkoku are full of that, though. Or any seinen from an author that has some normal social experience, especially at work, really.
The middle short hair cut similar to how his sister hair is. Maybe he got some interest with girls that have the same hairstyle.
I'm not implying he have a feeling for his sister, but when you used to that hairstyle, it can effect you unknowingly.