We have six kids, so I know a bit about this problem. Some times kids fight, because they want attention from the one they are picking on. It helps if you can arrange a time each day, to play games with them or do something special with them.
Sometimes they are unhappy about something. Telling them you are there for them and willing to listen, can go a long way to stopping the fighting. Another idea is boundaries, if they want you to give them boundaries, they will push until they get them. You can tell them no in a kind, yet firm manner.
Hey - my little brother is doing the same thing. When I pound him for it my older brother gets mad at me.My mom says its just that age ( early teens and 11 and 12 ). There's nothing you can really do, ignoring doesn't help nor does telling anybody else about it , he'll get over it.