Just some thoughts:
not all siblings like each other. But they are family. If you want them to learn team work and sharing, then teach them that. In light of their ages.
- Sometimes, there is always an instigator, who triggers the fights or irritations upon the other. So find out which kid is, triggering it. ie: with me it was my sibling. Why? She just liked to irk me and get me in trouble and she would deny, that she provoked, it. It was frustrating.
- Teach your kids the word "compromise." When my daughter was 2 years old, that is the first "big" word I taught her. She LOVED that I taught her that, a "big" word. And she understood. And she carried it out.
- Teach your kids about taking turns.
- Let them have their own time to themselves. Everyone needs that. Even adults.
- NOT everything, HAS TO BE, "shared." Teach them that. With my kids, I tell them that if they do not want to share something or it is special to them, then they CAN say so. In a polite manner. That alleviates a lot of "stress" on the kid who is "expected" to share.
- Teach your kids, that everyone has limits. AND teach your kids, HOW to say that. I taught my kids that when they were young. ie: if one kid has had enough of whatever, then they CAN say so. ie: "I don't want to, please stop." "I want to do something else now." "Can you please be quiet, it is too noisy for me." "I am tired, I want to just be by myself." AND the person that is being told that... NEEDS to heed to that. So, if a person tells the other to please stop, then that means, you need to stop. The person is SAYING their "limits" to things at that moment. If the other does not respect that and listen to that, then THAT is when fights or frustrations, occurs. And it can be avoided.
Nothing is more irritating, than when a person says to 'please stop' and the other doesn't listen and just KEEPS doing that irking thing to them. So teach your kids, about "cues" of the other and that they CAN say and express, their "limits." And then that is a BIG hint, that the kid has had enough. Their patience and tolerance, has run out.