Would that be too unrealistic in your opinion?

I'm planning to write a story about two siblings, a boy and a girl. You should know that the sister is four years older than the brother.

In short, both of them had a terrible life and, because of their bad parents, were mostly there for each other. The sister has cared for and watched over her little brother since they were children, in other words, their entire lives. He was practically raised by her. There's also a point where she tries to save him from a traumatic experience and succeeds.

My question now is, would that be unrealistic? Because girls, when they have little brothers, tend to have the urge to harm them as much as possible for no reason. Especially the experience, a girl should enjoy it rather than prevent it. She's playing more of a protective role, which is also very illogical.

Would you as a reader find that unrealistic?