I'm going to tell this kid to get away from my brother
What do you think you can say to this kid to get him to stay away from your brother?
I am going to threaten to hurt him.
I understand that you want to protect your brother, but it's not okay to threaten a kid in the process. Speak up for your brother, but keep the threatening out of it.
I'm going to threat him with a knife.
Using a knife to threaten someone could get you arrested since its a crime. You can warn someone without committing a crime in the process. Just tell them to back off and that you're going to protect your brother.