It could be that she finds the storylines or characters in girls manga uninteresting or not relatable. Sometimes, people have specific expectations that aren't met by a particular genre.
Perhaps she has had some negative experiences related to girls manga in the past that have soured her view. Or she might simply prefer other types of comics or entertainment.
Maybe she just doesn't connect with the themes or styles common in girls manga. Personal preferences can vary a lot.
Maybe it's because of some bad experiences she had with girls in the story. Could be jealousy or misunderstandings.
Maybe it's because you don't like the typical character traits or storylines associated with them. Or perhaps their art style doesn't appeal to you.
It could be because of misunderstandings or past conflicts. Maybe the girls have traits the boys don't like or vice versa.
The sister might hate manga hentai because it doesn't align with her values or interests. It could also be that she finds the themes or imagery disturbing or uncomfortable.
Maybe the boy did something bad to her or hurt her feelings.
Maybe the guy did something really bad to her. Like broke her heart or betrayed her trust.
Maybe it's because of some misunderstandings or personality clashes. It could be a plot device to add tension and drama to the story.
The uke might hate the seme because the seme is too forceful or doesn't understand the uke's feelings. Maybe there's a history of hurt or betrayal between them that causes the animosity.
Maybe he has some misunderstandings or she did something that really upset him.
Maybe you have had some bad experiences related to them or they just don't match your personal preferences.