I'm not claiming this as an original idea, but it's something we've been doing at our house to try and help with the sibling relationships. We have a large jar we call the "warm fuzzy jar" which the kids are trying to fill up with pom-poms. Each time I hear them say nice things to each other, or treat each other patiently, or share nicely, or let someone else take the first turn, or anything I tell them makes me feel "warm and fuzzy" inside, they get to put in a pom-pom. Whenever someone hits or gets in a big fight, or yells, etc . . . they have to take out a warm fuzzy. Once the jar is full, we are going to take them out to Gatti Town for dinner and games. I only use it for how they treat each other, not how they respond to their parents.
It was a struggle to get going at first, and it took about a month before I noticed an improvement, but I would say it has definitely helped the kids be more aware of how they are treating each other. We do have really bad days where we end up taking out more than putting in. But then there are days where they get on a roll, and I'm hearing lots of compliments and lots of "you go first", sometimes followed by a glance at me to see if I'll reward them for it. I tell them they definitely don't get to put one in if they ask to put one in, and so they just stare at me for recognition sometimes. It's not a perfect plan, but it's helped.