I'm going to sleep with my brother's best friend
How would your brother feel about this?
My brother will cry. But later he will feel better about himself because I will break up with his friend
You shouldn't be sleeping with people for any reason other than your love for them, and definitely shouldn't be sleeping with your brother's friends.  Wanting your brother to cry is wrong in itself, and you are also going to hurt his friend by using him so carelessly.
It's a game, no one is hurt and I want to see their reactions.
You're going to hurt someone just to see their reaction? Can't you understand how mean that is? I think you should stay away from your brother's friend entirely.