There's a lot of potential in that opening, but as the previous reviewers mentioned already, there are also some issues. The voice for me isn't YA at all (not that I read any, but I run into lots of YA in my critique group). And, of course, I read it when I was younger. This is not a sixteen/seventeen-year old, but a much younger child.
Actually, the incident is so effective because he's young. It's a moment of supreme disbelief, his world is shaken since his parents are not who he thought they were. It's a great point to start from but, yes, it's also tricky.
I wonder whether this scene could be a flashback to a younger self. But unless his parents are dead now, this won't get you much further either.
I also agree with Julie, there's a lot going on and you create pressure on a person we don't know yet. Since we aren't sure we care about him either, the urge of the paper wanting to tell him something isn't as immediate as it should be. Plus, I don't get crime vibes at all (I do write mysteries, so that's something I do know about).
Something along the lines of "A single sheet of paper changed my life" or whatever might be a better hook. Hm, doesn't work either, since you're using present tense. One way around that would be to write introductory passage in past, but tense switches are tricky especially at a point where you're still setting the tone of your novel.
Whatever you do, we need to get into your character first, understand there's a conflict that matters to HIM (and should in turn matter to us).
Can the steps to the attic urge him on? Can he ask himself if he should open the box? Does the box have to be in the attic, could it be in the wardrobe hidden under Dad's socks.? If he already knows he's getting close to forbidden knowledge you create suspense. Having him mull over the problem gives you a chance of introducing your character the same time as you hook your reader who is half wanting to prod you on and half-willing to tell the character to ease offf. That sort of setup will give you conflict, hook, and character - and the inciting incident that sends your character on his journey. And this incident might well have happened in the past and is influencing his present.