I'm going to say I don't want my brother to see an r rated movie
Why don't you want your brother to see an r rated movie? Is he old enough?
He's not old enough, but he's going to watch it at a friend's house. I don't want my brother to get access to that kind of stuff too early in his life.
It sounds like you care very much about your brother. Tell his friend's parents that your brother isn't allowed to watch R rated movies. He may be upset but you are doing what is best for him.
I don't want to do that. I just want to tell my brother not to watch this movie because I don't want to be a bad brother.
It is good you tried to warn him about the movie. Maybe it would be better for your parents to step in because they might not be aware of the situation.