Ummmm

This doesn't sound like a great idea

There seem to be a lot of potential negatives to telling them, but you haven't explained what you hope to get out of it

Why do you feel the need to tell them? They do not have to know everything about you especially if they cannot be trusted. You are already preparing yourself for them to fail which means you are fairly sure how this will pan out. If you want no one else to know, the only way to prevent that is to not tell the people you do not trust. I would give them another test to see if they are worthy of being given this information. Something that won’t hurt you but something important enough to test them, something you don’t mind the world or the rest of your extended family to know. If they pass that, then consider whether they can be trusted with this information. If they fail, then you know they have not earned this knowledge.

I am afraid that they will fail.

Telling a bad actor NOT to do 'x' is a great way to get them to do it.

I wish you well in this difficult portion of the quest.

You are their Son. That's all that matters to them, trust me.

I'm Aspergers and have three kids who are in the spectrum. Use it as an excuse to succeed.

OK. But what's the upside here? Is it some kind of 'win' thing? Retribution? Revenge?
Here's another option. Don't say anything and handle your own shit. You're a grown man. There's no need to seek out drama when there's no benefit to it. Just two cents from a stranger.

A narcissist can't care about others. So testing them only invites more hurt. If you need to tell them for your own sake that's fine. But don't ever expect love or acceptance from a person that can't feel that emotion for anyone other than themselves.