I don't think it's really unbelievable. If she wouldn't have a very soft spot for Takayuki herself, she wouldn't have directed her hatred towards Mitsuki alone. She would try to protect her precious sister from a two-timer then!
Still, he thinks of him as onii-chan - why? That she cares for him is very obvious (ep5, teary Akane begging Mitsuki to take care of onee-san AND ONII-CHAN), the question is how far this care really goes.
I guess that's what eps 6-14 are for
