Just go on about your business and pretend you don't notice he isn't talking. Talk to him like you normally would and if he does not respond, answer out loud for him. He will give up long before you do!
Could it be that he isn't sorry? Did his sister do something to him that made him think that he couid hit her? I would never make anyone apologize, especially if they aren't sorry. I don't apologize if I am not sorry. There isn't much you can do to make him talk. Just don't do anything for him until he talks.