Most common source of argument between sisters

Hi Sweetheart, My sister and I are 17 years apart in age, so there was never much fighting there. She acted like a brat, and I would bust her butt. LOL My daughters however, are only a year apart, and they fight like crazy. I remember one funny thing that I will never forget. They were in Kindergarten and First grade. Kristen, the oldest, had let her sister borrow a sweatshirt to wear to school. As soon as they got off the school bus, I could tell Kristen was mad at Lauren! She was stomping and kept looking back and saying something to Lauren. They got into the house and when I asked what was wrong, she said "Mom, just look at what she did to my sweatshirt! Do something with her!" Lauren had dropped spaghetti on the shirt at lunch and there was red marinara stains all the way down the front of the light pink material. Of course, I knew it was something I could get out in the laundry and so I tried to defuse the situation. Kristen was standing there...arms folded and she was tapping her foot, just like I do when I get angry. She was so hysterical that all I could do was laugh, which made her even angrier. She was 6 years old, but was acting like she was 16. I knew then that the teenaged years were going to be crazy, and I was right. BUT! Let someone from the outside come in and verbally attack one of the sisters, and the other one will be on them like a wild animal. They may fight, but they are very protective of the other one.