I think it's great that you're even thinking about the financial fairness of the situation. I don't think it will be an issue unless the other daughter can't do an activity because of her sister's gymnastics--whether it's due to money or time. And I do think time is a big issue to take into account when balancing family resources. My husband still remembers most of his childhood weekends spent at gyms watching his older sister's meets that we've decided one parent takes the gymnast to the meet and the other parents does something else with the other kids.