When to reveal this plot point?

My novel is about a girl who killed her brother, believing he is a murderer himself and wants to do it again. The boy, who the MC believes is her brothers victim, was actually killed by her therapist (he was also the victims and her brothers and her own therapist) who raped both her brother and her apparent victim. The story is about the girl investigating her brothers apparent victim and slowly figuring out that her brother is innocent.

My question is: when do I reveal that she was the one who killed her brother? (his death was written off as a suicide)

I work with the 4part plot structure