I do not get your logic.
It has been made really clear that he went crazy after his sister went missing, meaning that there is a probability that he is a crazy psycho right now. If he figures out that she isn't his sister, who knows what might happen to him.
Plus she is with him right now so I don't get what you're complaining about. I don't get why you're calling her stupid. I said that she is trying to prevent it, you came about and said that it's easy to prevent it. Idk what you want from her, she is doing what you want her to do and yet here you are, insulting her. It doesn't make any sense.
Is it just me that feel annoyed about the way she think? I mean, she knew the brother started killing people when he lost her sister, and here she is thinking the brother will kill him.