I'm almost finished reading, "The Secrets of Happy Families," and I really like a lot of the ideas in it. You might check that out.
I'm not a fan of "Siblings without Rivalry," although I see it mentioned all the time whenever this comes up. It advocates letting kids figure it out on their own, and at one point said that parents should acknowledge and act as a sounding board for a child's very negative feelings about his sibs. I disagree with this advice because I don't want any of my kids to think I won't defend them if someone (even a sib) says something mean about them. -- Just my take.
I'll admit we haven't had a huge issue of sibling rivalry (twin boys and a girl), but I see this with my nephews. An observation I have about them is that their parents ignore their bad behavior until it's a crisis and then intervene with yelling and punishments. If it were me, I would stop it at the first bicker by asking them to speak kindly to each other. A big conversation we have had is, "Is this the kind of person you want to be?" "Are you the kind of person who would…."
I also think when bickering or mean behavior gets out of hand, it can be because there's not a clear manifesto, if you will, of who your family is and what you stand for. Can you get your kids to buy into/contribute to the idea that you're a family where you do.. (positive attributes) and where you support each other in attaining those goals?
When bad behavior strikes, I get a lot of mileage out of being shocked. I guess if it happens frequently, you can't really be shocked, but I have said things like, "Wow… I'm so surprised to see this because you're such a kind person. What makes you think that's OK?" I want my kids to think I know they're kind at their core and that bad behavior is not who they are-- seems like if I do that, they don't want to disappoint me.
And a final idea-- I read once that siblings who are friends as adults have memories of shared activities in their childhood, which creates a basis for their ongoing relationship. I think the book said the adults mostly don't remember momentary bad behavior, but do remember the good times camping or building legos, or whatever. So if you can create common interests with your kids, I think it will ultimately be OK.
