Brother's horror reasoning: What horror did my sister think of?

My brother suddenly went crazy and killed the whole family. Among them, my sister was lucky to survive, but she lost her memory because of shock.

Losing parents and memories, my sister who survived like a walking corpse met a fortune teller one day and began to divine her past.

"Why does my brother suddenly go crazy?"

"No, your brother is quite calm."

"Then why did my brother kill the whole family?"

"No, your brother only killed one person."

Suddenly, my sister remembered everything and began to sob.

Excuse me: Why did the fortune teller say that my brother only killed one person?

Are you talking about this topic? In fact, it was the insane sister who killed someone. My brother was executed to protect his sister, so he committed suicide.