Conflict Conflicted

I got a Facebook message from my niece. She wanted to know why her aunt, my sister, hates her. She told me of something despicable that my sister had done to her that amounted to online bullying. I was mortified. My sister can be down right mean and expect everyone to be OK with it. I wasn't OK. After spending the day trying to let my niece know that she should just so totally disavow the actions of my sister and not let them tear into her fragile self esteem, I did something stupid. In the same cyber-bullying way I let my sister know that what she had done was wrong. Well, to tell you the truth my sister, who doesn't ever see herself as being wrong, only wronged, decided that I was wrong to "attack" her. I ignored her first phone call because I knew that she'd be angry and spewing. I wanted to give the situation some time to diffuse. She called me a week later and I took the call. I explained to her that what she'd done was mean and had deeply hurt my niece. She tried to lie to me and tell me that she hadn't done it, but it is obvious that she did, and this is always her first defense mechanism. I didn't buy it. I pressed her and she fell back on her second defense mechanism which is to become a profane name caller...real mature, I know. My therapist calls it emotional backmail. She was trying to dissolve my high ground by making me seem less. She is very good at it. This time I took my therapists advice and cut her off. I sternly told her that the conversation was over. The truth is that I am unwilling to hold another one with her. I know that as a Christian I am supposed to forgive as Christ forgave me, but does that mean that I have to continue to expose myself to her toxicity. We all cut one of our brothers out of our lives because he was so very toxic no one was able to withstand his toxicity. My sisters is minor compared to his, but she tears down my self esteem with her meanness. I really do not want this to be in my life any more. It's not that I don't want her in my life. I just don't want her meanness and blackmail in my life. But cutting her out of my life feels like unforgiveness to me. In Christ we are called to forgive, and this seems like the antithesis. I am conflicted.