It's actually really nice to see more of her more embarrassed, almost vulnerable sides the past couple of chapters. Sometimes teasing female leads feel too enigmatic to the point it's almost hard to connect with them but this shows she's just an awkward teen like any other.
I remember Ishigami commenting on Chika's perfume in one episode of Kaguya Wants to be Confessed to. If you want to compliment someone's smell, just tell them they smell nice while breathing heavily.
It may seem strange she's not telling her big sis about Kubo, but it's not like she'll be able to see him unless she draws attention to him, so it would just come across as odd.