Jitsu wa Imouto deshita. ~Saikin Dekita Giri no Otouto no Kyorikan ga Yatara Chikai Wake~ - Vol. 1 Ch. 5

[Nayrael]

I wouldn't say that there was a drastic personality shift. Rather, she realized that he was not actually flirting with her and would stop now that the misunderstanding was cleared. But since she clearly wants to keep the relationship rolling (rather than become a proper sibling), she is now taking the initiative. In a way, she is imitating him (or him as he looked in her eyes).

 

[Equilibrium]

Remember to say no homo when you nut in your bro, that makes it okay.