North,
It's just my opinion, but I think the most valuable quality your girls possess is leadership. The more experienced girls teach and support the younger or less experienced girls. When one girl is doing something incorrectly one of the other girls is often the first to help her out. They play for the team, but they also play for each other. The team has some natural athletes, but it also has strong leaders, hard workers, and eager learners (OK not all of them are all things, but there is a good mix).
They have high expectations of themselves and of their teammates. I wouldn't be afraid to bet that a girl showing up on that field not ready to work would hear about it quickly and loudly from several directions. They aren't mean or cruel, but they don't cut each other (or the coaches) any slack either!
I hope you know that the work you do with the team and with their little sisters is greatly appreciated!