Besides dd, our gymnast, we also have a ds, her younger brother by about 4.5 yrs. He does his sports, travel soccer, wrestling, basketball, right now he is playing flag football. My rule is games before practices, so if he has a game the night of one of dd's practices (which is 5 days a week), I go to his game. If he has a game and she has a meet, then dh and I split up and dh usually takes ds (dh is ds' soccer coach) and I go with dd.
As for vacations, we do usually go when the gym is closed but that is not a big deal. I just arrange for that time of the yr, I don't announce to everyone that it is when the gym is closed. In the future our vacation times may change due to ds's schedule, but that is not the case right now.
I believe that my ds can only resent things if I make a big deal of it. He is young enough not to notice too much (8.5 yrs old). Dd and I went away to a flyaway meet this yr and dh and ds stayed home. Dh planned a couple of special things with ds, including going to Dave & Busters and out to dinner where ds wanted to go. He loved it and didn't miss the fact that he didn't travel with dd and myself. Actually dd was a little jealous of ds, as she really wanted to go to Dave & Busters!
I dont' have to worry about staying during gym as the gym is only about 15 minutes from our house and with practice being 4 hrs long, that is not an issue. I will go back to the gym about an hr before it ends and sometimes ds has to come, but it's not for a long time!
The good thing for me is that dd's sibling is a boy and he was never involved in gym. I can only image how hard it would be if her sibling was a girl and would probably want to follow in dd's foot steps - that would be a problem. We could not afford to have to in competitive gymnastics. Again, luckily I don't have to worry about that.
It's all about the approach. The gym is a part of our lives - all of us in our family, but you don't have to make it appear like it rules our lives. We are the adults and I keep the financial aspect and the whole vacation set up amongst ourselves and let the kids be the kids. I am sure as the kids get older they might be more in tuned to things, but I can't worry about that now - can only worry about the here and now!