M.M.
You could try asking questions like, "Tell me one or two things you'd like for her to do that she doesn't do and/or one or two things you'd like for her not to do that she is doing." I might actually ask him to draw his answers to this question. I am a child therapist and I find that kids that age express things, especially things they may be afraid to talk about, much better through art and play. You could also observe his play to see if there is anything "off" about the interactions of the people/toys in his play.