My two oldest are just shy of 9 years apart, and then 2 more follow in close succession, but we have trouble with the oldest (who is a 14 y/o boy) teasing the others (5 y/o girl, 3 y/o boy, and 6 month baby). It's like he gets some power trip by teasing them, or it makes him feel in control some how. We have reprimanded the eldest and sent him to his room until he could behave properly and that has helped. We also have make him write sentences ("I will not tease my sister"), 25 to 50 (he HATES to write sentences) because he knows better. He sometimes treats them like they are there to amuse him. When the fighting is 2-way, it is sometimes effective to have them sit in chairs and face eachother for 5 minutes. I always make them apologize and hug and say "I forgive you". Hope this helps!