I think all of Amy's suggestions are great, but I'd add a few thoughts. I do think the little ones can manipulate the situation to get their older sibs in trouble. So, I'd make this a win/win or lose/lose situation. When you are both being kind and getting along, you both earn extra computer/tv/friends (whatever the child's "currency" is). If you are not, you BOTH lose. This motivates them to get along better and to work together to improve their relationship. I'd also quit trying to talk about it so much. Sometimes, we inadvertently reinforce the very behavior we are trying to stop by paying way too much attention to it. So, I'd take action and reduce the talking about it. So, I'd describe to your children as "If you two disturb my peace by bickering, tattling, calling names, not sharing or any other poor behavior, you will both lose out." When I began to look at it this way with my own kids, things really began to improve in their relationship. Today, they are both young adults and even socialize together!