Look, I know she's a bitch, but she's actually agreeing with Itsuki there.
She also believes he's too nice to be related to her, thus, he is adopted.
Her bridge comment isn't implying he's some kind of troll offspring, or whatever.
Ok but the problem is that he's normal/chill, which implies one of two things:
1. Their parents failed with her, given she acts like that
2. They are normal parents who raised their son properly, making her the adopted one
Idk atp, the longer she stays in this story, the more agitated I get, so if I look crazy with what I'm saying, feel free to blame that on the braincells I lost thanks to her