First you need to digg deep to see things from his perspective and acknowledge to him that it is how he is feeling, then the conversation will open up more. (Empathy), next try and restart the conversation by apologizing and ask a question that gets him to say no, so he feels he is in control. Then follow it up with an open ended question like “what is your sister supposed to do?” because he felt some sense of control from the previous question, he may concede something on his own. Make sure you prepare the questions and not wing it.