I think I need to break off my engagement because of my fiance's brother
Why do you think that has to do his brother?
His brother is so mean to me.
You shouldn't let your fiance's brother be the reason why you don't get married. You won't see his brother all the time so why does it matter that much?
His brother is a big influence on him though.
Well if you love your fiance you should really think about this and not do it lightly. If you really love him and want to have a life you should marry him and not let his brother ruin your relationship. You could try to talk to him and get him to confront him about the behavior.