Sibling rivalry is a concept that has been around since the beginning. Whether it was Cain and Abel, Romulus and Remus or more recently Peyton and Eli Manning, the idea of sibling rivalry will always be a setting in any competition. For Krysta and Sadie Zbikowski, sisters who compete in cross country and track and field for Bridgewater State and Fitchburg State, sibling rivalry has nothing to do with it.