I told my brother when we were kids we had a secret sister. Later on I told him it was just a joke.
Why would you do that to him?
I was wondering if he could act like me.
It's okay to play jokes on your sibling from time to time. As long as it doesn't hurt him mentally.
You are not understanding. It didn't hurt him in any way.
If he didn't get hurt that's fine but you are getting close to that border line of jokes that are mean and unfunny. You should do more light hearted jokes.