All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
As long as your fiancé is a righteous and good man, we believe that the mere disputes between him and your brother are not a valid reason to break the engagement, especially if your brother is the one who mistreats him. If you break the engagement for the sake of your brother this time, you have no guarantee that he would not mistreat any other future suitor of yours.
In any case, you should know that retaining your engagement does not constitute any transgression on your part against your brother nor does it count as a failure to uphold good relations with him. Hence, it is advisable that you take it easy upon yourself and retain your engagement; you should seek the help of Allaah to rectify the relationship between your brother and your fiancé.
Allaah knows best.