We have the same situation at our house. My guys are 11, 8 and almost 5. It is usually the middle one who is left out and he is very sensitive and easily hurt by it. Talking to them about it seems to help temporarily - none of them wants to be mean, it's just that the oldest and the youngest are the most alike and seem to gravitate to each other. They also seem to go through phases of getting along just great and then wanting not much to do with each other...usually it's somewhere in-between. I just keep preaching kindness, empathy and the always popular "you have two brothers who love you, not just one!" speech to whoever seems to need it at the moment. Family life is (in my experience) always a little bit sticky.