DuckLover2399, I'm sorry for your pain. I will pray for you and your animals.
Try this: Google "serial killers abusing animals". It will bring up a bunch of articles about the link between abusing animals and becoming a mass murderer. Print out a few good ones and leave them around where your parents can find them, or if you can do it calmly, just hand them to your parent(s) nicely and tell them (try to sound sincere - are you a good actress?) that you are worried about your brother's future. Hopefully it will scare them enough to do something about it. If they try to brush it off and tell you that you are being overdramtic, or something like that, just look them squarely in the eyes and say clamly, but firmly, "Are you REALLY willing to take that chance?" I can almost guarantee they won't know what to say, so after a few seconds, just turn around and walk calmly away and let them stew over it.