Bi-Polar Sibling Love...err hate!

I feel at a loss when it comes to my two girls sisterhood. One minute they are the best friends (usually when Nanna is the director of play) and the next they are battling it out. I never know when to intervene and I often wonder if by trying to help, I am making it worse. Please leave suggestions, insights, or any medication that might stave off the manic swings in my children's play patterns.