He could sue your sister for his injury: for any medical costs, for any lost wages, and *possibly* for pain and suffering, if it was a severe-enough injury. Under the circumstances you write about, it would be likely that if he sues, he would recover these costs (your sister or someone in her household was negligent, or careless, in not locking the cage).He could also report the incident to the police. If there is no prior record of attacks, then given the circumstances (he was in the house, where dogs are often territorial), it is unlikely the police would take any action at this juncture, other than to make a report for future reference.