Is it wrong of what I "did"?

I was invited to a kids birthday party by my brother's ex or current girlfriend. She invited me months ago. On my way there, my sister told me she and my wicked mother was invited as well. My mom didnt want to tell me because of the girlfriend's bad influence on me. I'm 26 and I have a daughter so there's no need for negativity and treating me like a child. I was told by my sister that my brother hit her or she hit him. Immediately, I left the complex because I didn't want to be involved or know what happen between them. This girl calls me and asked why I left. I felt bad that it was rainy and no one showed up but 5 people. So I came in, ate and left in 2 hours. But before this, this girl had a black eye and a bruise on her arm. Now she revealed everything and I said my sister told me that she was hit. That day my brother got mad for no reason and she wasn't in a good mood after work that day. After banging her door window, she pushed her door to him, fought in front of her kids and just hit her in her face. Anyways after I left, I felt something was to happen. I got home and my sister tells me she got yelled at by my brother and mother. I called my mom asking was there a reason to go off on my sister. I was told dont tell her what to do and stay out of her crap. My sister and I r backstabbers and big mouths. She was basically defending my brother who's a bum, sells drugs. I stopped trusting my mother at 14 because it seems she loves him more than my sister, more than I. 5 minutes later my brother calls me, tells me to stay out of his business and crap. Then sends me a text saying stay away from the family, mom and called me a bitch. My husband told him to not contact me ever. Thing is, there never was a family. My mom called her baby to fight with me. Later my sister tells me my mom is not mad at me but upset. She didnt expect me to be out of her life. She was also crying on her way home because I don't want to talk to her. Honestly she had it coming. She and my brother did me a favor. Point is, my 17 year old sister told me of a domestic violence and my mom and brother yelled at her for something she did right?? She told me an abuse and it was obvious, this woman had bruises on her eye. So....... exactly what I do wrong?