What should I do if my in-laws mistreated my parents, causing them to want to end my engagement, but my fiance and I are unwilling to give up because of our love for each other?
You should tolerate the mistreatment if any and try to avoid any situation or act which instigates unpleasant relationship with your in laws.
Try to explain to your parents that you and your fiancée are in agreement and his family mistreatment should not ruin your engagement.
Wassalam.