Sure it's been established at chapter 6 that they're step siblings, but it's pretty clear that the brother really cares for his sister like real family, despite how 'devilish' she is. If he's been holding himself back for that long, she has gotta understand that she's gonna need to take initiative and get her feelings across first before they get anywhere. If the brother succumbs to her seduction, he's only going to hate himself for it, making their relationship much worse than she wants it to be.