What can I do about my step brother?

My step brother has a dog and just bought another from the same litter for his girlfriend. I know from past experiences that he isn't a good pet owner. And when I say isn't good, i mean abusive 150%. He was just at Our parents house yesterday and he was talking about his girlfriends dog. He said he threw it into the wall, and bunches it in the head as hard as he can as it's punishment for doing something bad. The girlfriend does have a problem with it, but she's also pregnant with my step brothers child. Let us not forget that his girlfriend already has a son. So here we have a bad situation getting worse. Back to the dog. The girlfriend said that last week the dog had been so abused that there was blood all over the kitchen from my step brother beating it. This isn't a big dog either it's a little dog. And my step brother also beat it like this once for it pooping on the couch from eating his cough drops. I mean the dog ate the whole bad of them and couldn't breath. I told my step brother that if he took the dog to the vets then they'd open a case on him, he said he wasn't taking it and he wouldn't ever be taking the dog. He said it was past it's shots and he didn't care he wouldn't take the dog to the vets. And he continued to say that if the dog had a broken limb then he would just snap it's neck. Now when he said that the only thing that came to my mind was.... He had done that before to another animal. It had been a cat, and a perfectly healthy one. But he had done it before. My step dad told him they would take the dog, but they said no. So my step dad told him if he hears about it happening again then he would call the police and have them look into it. My worry is that he'll continue to do it, but just won't tell his dad. We don't talk to the girlfriend enough to know if it's still happening, and my goodness you should see him when he talks about it. He gets so mad, like he's there all over again. He shakes, his face gets all red, the veins on his neck stick out. It's completely freaky. I don't want to go behind my step dad's back and tell the authorities. But i fear that this dog is going to end up dead. **** he's even gotten the dog high off of weed.