I am going to tell my brother that he should not tell anyone about being gay.
What is your reason for wanting him to remain in the closet?
I just don't want other people to know he's gay.
Your brother's orientation is his and has no reflection on you in any way. Never force someone to do something just because it affects your feelings.
He should stay in the closet because he is my only sibling.
If your brother is gay than he should be able to come out if he chooses to. You have no right to tell him when he is suppose to let the world know about his sexual orientation.