To be honest, my kids get along pretty well. I'm not saying that to brag or anything, I just think the fact that I don't discourage my kids arguing with each other needs to be put into context. So maybe someone in a different situation would have a wildly different view of things but, from where I'm standing, I think there are times you should let siblings fight.
I'm one of a whole bunch of kids in my family, and while we rarely got physical our house was still... well, let me put it to you this way: have you ever seen footage from Parliament in the United Kingdom? Yeah, it was like that. One person would be shouting while a whole bunch of other people would be screaming whatever they thought about what the other person was shouting about, and everyone was just trying to be as loud as possible in order to make their point. It was a charming kind of chaos.
I'm the oldest, so I often felt it was my duty to take things in hand and mediate disputes. As I grew up, that carried over and even outside of my family. I would often find myself work to maintain social harmony in whatever setting I'd find myself in. And now that I have kids of my own, that instinct is still very much alive and well. I've worked hard to suppress it, though. Blessed are the peacekeepers and all that, but I've come to learn that a little conflict (and learning how to resolve it, sibling to sibling) is good, not only for kids but for meddling mamas who need to let go of the idea that they are responsible for everyone else's feelings.
Of course if things get physical, dangerous, verbally abusive, or just extremely out of hand, by all means hop on in there and mom it up. But here are the times when it's OK to take a backseat: