@Amizer it said she has been behaving like that with him for 9 years, ever since both of them were infants. Back then they hadnt grown up together, and I guess mangaka thinks it makes sense for an infant to be in puberty (and remain as such for 9 years). If she has been in puberty and spent the 9 years avoiding him because of that, I can see why the fact her 9 years of life (all spent with him) wasnt enough for her to see him as a brother.