Rough Go

My middle son is having a rough go of it at the moment. I think part of it has to do with the fact that for the majority of the time he is the middle but at times he is the oldest. My oldest son is my step-son, so every weekend and every other week he's at his mom's. This causes for a lot of jealousy when both boys are home together. It's gotten so bad that my oldest resents my middle son's friend. They fight and they fight all the time over pretty much everything. My oldest can of course be down right evil, as only a teen can be, and that just crushes my son.
The oldest and my daughter team up and pick on my middle boy. Unfortunately he gets so hurt and frustrated by it that he usually melts down. It's truely sad and I have no idea how to stop it. We've tried grounded them from each other. We've tried taking things away. It got so bad once, on the way to SF for an aquarium and natural history museum I pulled the car back into the driveway and made everyone get out and started unloading the car. That worked the best. The stopped fighting amazed that I was serious and after about 15 minutes came and asked to go and promised to stop. Well we still had attitude issues on and off but at least the constant picking stopped.
My hubs and my son are exactly alike, and that is causing problems, and the other two pick up on this. My hubs and my son butt heads like two bulls jockeying for position. I have finally told my husband to knock it off and figure out a more peaceful solution as I am done done done being in the middle. I will no longer referee a fight between the two of them. I've also reminded hubs that he is the adult.
I am glad that school is starting soon. While I like summer and being more relaxed and freer to do things, I am going to be happy that there is no more constant bickering and nitpicking going on.