Usually I let them figure it out on their own, with “choices”. Like, “you can disagree with each other but the rest of the family doesn’t need to hear it.” Or, “I can see you’re both having a hard time. Why not NOT play together for a while?” I find that if I take sides, whatever side it is, trying to solve the issue, the peace bought is shallow and short-lived. I’m more stealth than that. I find jobs they have to do together (dustpan and broom) or rewards they can share (popcorn if they beat the timer cleaning up an area together). I school them together, allowing for happy banter while we use scissors or markers. I might even whisper, “Pssst….let’s surprise your brother by making a few paper airplanes. He won’t even GUESS who did it!”
I think our children have good relationships with one another. With six children, there are multiple relationships always going on at the same time (each child having a different dynamic with each of the five other children). It pleases me to see them interact with one another; I’m confident that they will be very close as adults. After all, when you spend all day every day with your brothers and sisters under the watchful eyes and ears of loving parents, it’s hard not to just let go of the dumb thing argued about and just get out a board game.
Quantity time with one another is the best relationship builder. Are your children spending enough time with each other?