My brother wants to propose to his girlfriend but I want to ask him to wait.
Why do you want him to wait to propose to her?
I want him to think about his decision more. I really don't think she's right for him and their marriage won't last.
You shouldn't butt in your brother's choice on who to marry. He's an adult, so respect his wishes.
I don't want him to make a mistake that he'll regret.
I know you think you know what's best for your brother, but he may really be in love with his girlfriend. Talk to him about your concerns but ultimately he has to make his own decisions. Try to support him and wish him well.