To me this feels about due. She showed up wanting to kill the girlfriend, pivoted to playing along with the mistaken identity, has been going through a painful retrospective on her brother where she realizes what she's lost / never really understood. She might lose the person she thinks knew her brother in a way she didn't. The last person who shared her brother. Plus whatever happiness she might find now in this routine is as her brother and not herself. If she reveals who she is right now she actually has nothing. The mental last straw is often disproporation to the action that triggered it when you bottle stuff up like that.
Fortunately there is at least one person who know the deception and might call her out before she totally blows this.
I like how the author draws the way the brother answering, from her perspective, his words may feel like criticism or something bad towards her. But, in reality, from the looks of it, he is simply telling her, she can't be him, she just has to be her. Welp, this is my interpretation.