I left a secret at my brother's wedding
Why did you keep the secret?
It was something I wanted to tell him but I know it would ruin the mood, so I left it for later.
If the secret is important then you should immediately tell your brother. If this has to do with the wedding then it cannot wait.
I was going to tell him, but I'm going to wait till after they have a kid then I will tell him, I feel like it will go better then.
I do not know what you are keeping from them, but I would get it out of the way.  Things will only be worse down the road if you hide it until later.