Why does Animal Control say there's nothing they can do when my sister was brutally attacked by her roommate's dog

She can call and have a police report filed. This will have many ramifications though. It may effect the current living arrangement through landlord-tenant law, personal injury and perhaps criminal defense. She should consider consulting with a local attorney. It will depend what the goals of your sister are though however. Whether the purpose is to have the dog removed from the premises of the apartment, cost of injuries or whether she feels her roommate should face criminal charges for it. These would all be things to consider when talking to local counsel. Best of luck and hope your sister is okay!