Having difficulty getting along with my future fiancé.

My fiancé and I have been together for five years—I'm 25 and she's 25. We're planning to get married next year. Recently, she has been rekindling her relationship with her brother, the same brother who has physically hurt her for simply resting after college, and for not doing house chores. When she asked a friend of his to take down her picture from social media, he refused, and her brother didn't support her. I stood up for her, even getting into a physical altercation with him. Now, I find it hard to accept that she has patched things up with him, as I view him as my enemy. I'm deeply in love with her and can't forgive anyone who has hurt her. I've been trying to explain why it’s important for her to keep her distance from him, as I fear that if he mistreats her again, it could affect our relationship. I’m feeling overwhelmed and confused about the situation. What should I do?