What should I do now?

My older cousin got physically abused by her boyfriend today. I didn't know at first, my younger cousin told me to call the cops. I said no, they are just yelling, not knowing the truth. I realized it wasn't the case as I heard her scream: Babe, stop! Why are you doing this? I was frozen with shock. My cousin then began freaking out. My cousin ran inside and locked the front door. She called her older sister and told her: Don't let him in the house! He just tried to choke me and threw me around by my arm! I started calling and texting my aunt immediately. She didn't answer. She came home and found out. She tried to call the cops but my cousin said no, I don't want to talk to the cops. My cousin left to home. And now my younger cousin is seeming to resent me for not calling the cops. I don't know what to do. Should I have called the cops? Would he have stopped if I did? Would he have killed her or me for "snitching"? Would anything I could've done changed it at all? Would the police even believe a minor? What should have I done?