Sometimes, she thinks she knows. But he's always so uppity-up and happy-go-lucky, and it makes her feel a little bit selfish for thinking that it's because of her. She tries to forget about it sometimes, but it doesn't always work because he's always with her, around her, watching over her, protecting her. And friends are always telling her she's a little blind not to notice.
Sometimes, she thinks she notices, but then Onii-san tells her about his and his little excursion in France, and she spends the next few weeks debating over whether or not to dye her hair blonde.