I need your help followers! Prologue or no Prologue?

So I've posted a bunch of times about the new book I'm beginning to write. If you haven't seen any posts about it, look around, you'll see them. Just scroll down like two or three posts and you'll find one about it.

I want this story to be mystery-ish. In the story my character discovers a really serious family secret. In the prologue, I wrote the scene where it happens. But, since I want it to be a mystery, should I not include that? Because then the character AND the reader won't know what's happening.

Or should I leave it in?

CONFUSION! HELP ME PLEASE
Thanks darlings <3