K.
Yikes, sounds irritating to say the least but I recommend trying not to foster your feelings of frustration. I think that only compounds things because he picks up on the fact he is rubbing you the wrong way and it makes him feel even less secure perhaps. The less secure he feels than the more annoying he will act. It can be a vicious circle for him that I would try and help him break. Try and take the emotional element out of it for you and just keep talking him through it. I wonder if finding an activity for him that he might be good at. Maybe he is already in some type of class but if not maybe some type of swimming lesson, or martial art, or art class that only he does and not his sister. We all have bouts of insecurity and he is going through one. I would put into words for him what you think he might be feeling... "you know it can be hard sometimes having a sibling and that you know he sometimes feels left out or not as good." Tell him that the two of you are going to work on making him feel better about himself and that you guys will figure it out. Good luck!