It's not the moments whenever he's speeding, or engaging in daredevil acts. Rather, it's when he gets into fights for her sake that drives her to tears: the frustrated kind. His anger is something that's far more dangerous than her own. Apart from this, being separated from him also triggers inescapable sadness that she isn't used to dealing with. She's well aware of the fact that the co-dependent, incestuous relationship she has with her brother is dangerous, and not socially acceptable, but she's also aware that he's the only one who can remind her that she is human.