A friend of mine found this old article from Dear Abby. A woman had written to her to tell her how she solved a problem at home. Since music always seems to have a calming effect, the woman asked that her two daughters SING their complaints about each other to her. As the started to sing the complaints, the anger seemed to melt away and most of the time they would end up laughing.
I think I will try this with my two daughters as well!