Hi honeymommy,
I can only imagine how frustrating and painful it is to watch your kids fight with one another. Please do not feel discouraged! I think the fact that you are reaching out makes you a great mom!
I'm currently a mom to be, but from experiences with my nieces and nephews, it is natural that as your kids grow, they will have changing needs which affect how they relate to each other. Sometimes, kids have a strong concept of fairness and equality and might not understand why siblings of other ages are treated differently or feel like one child gets preferential treatment.
Easier said than done but if at all possible, try not to get involve unless things get out of control and if there is physical harm. If you always intervene, you risk having the kids to start expecting your help and wait for you to come to the rescue rather than learning to work out the problems on their own. At the same time, you may inadvertently make it appear to one child that another is always being "protected," which could foster even more resentment. By the same token, rescued sibling may feel that they can get away with more because they're always being "saved" by a parent.
Recognize when kids just need time apart from each other and the family dynamics. If you do get involved, my advice is to separate the kids until they are calm. You could also set up some ground rules on acceptable behaviors; i.e. no yelling, no name calling, keeping hands to yourself, and introduce consequences when they break them.
Do try arranging separate play dates or activities for each kid occasionally. And when one child is on a play date, you can spend one-on-one time with another.
I think we just need to keep in mind that sometimes kids fight to get a parent's attention. In that case, consider taking a time-out of your own.
All the best and good luck!