OH MY!!! Siblings at war! In most cases cats (especially multi cat homes) form a hiararchy, and one will scrap with others to become top cat. Sounds like that's what's happening with your guys, and it will continue until they settle their disputes, and one becomes "KING"! If it gets really aggressive, arm yourself with a squirt bottle (say nothing during the spat) and squirt them to break it up. Call it one of the joys of domesticating the wild cat!