Wut? Arn't you brother and sister? You already said it's normal for you to come in the bathroom when your brother is taking a shower (Or there is some memory's missing in the MC brain )
...What? The only reason that would explain her reaction would be either her not wanting him to enter the shower for:
A: She was about to shower and either doesn't want to be seen by him nude, or has something on her body that she want's to hide.
B: She wants to hide something/someone in the bathroom.
C: She thinks he wants to masturbate in the bathroom... (Okay ...Wtf is wrong with her....?!)
D:It was a joke, pun or saying that was lost in the translation.
œž: It had no meaning. (...?)
???: Bullshit, stop thinking, stop using your brain... don't try to make sense from some low quality shit with only a good setting...