Not mentioned in your story is birth month of your two daughters, and how that aligned with the sports season. Its been shown that there are a greater percentage of summer birthdays in professional baseball, as an example. Older kids in the age group (particularly at young ages) have a developmental advantage going in compared to younger kids, which can result in more reps, more opportunities to improve, and more playing time. This all compounds over the youth years.
I have read about that too. In my situation it's backwards. My older daughter is Feb, younger is Oct. In fastpitch the season turns with the calendar, so the Feb birth month would have the advantage.