I think I might "unwittingly" let my cousin know they're adopted.
What? Why would you tell them that?
I love my cousin a lot, but family is a weak spot of mine.
It is wrong and hurtful if you tell them they are adopted and no one else knows. You should respect their parents not telling them unless the kid asks.
I think it will have a beneficial effect on my cousin. They will mature faster and will be happier knowing the truth.
It's not your place to tell them they are adopted. The parents know best how your cousin will react, and your cousin will probably react better hearing it from them.