In all actuality, IMO, only persons boarding the ride should get into the lines. Trust me, I have nothing against baies, I've been off a little over two years from raising a sister 17 years younger than I, not to mention my friends' kids, neighborhood kids, kids I've kept and keep. I like 'em all, I remember the people that were around me when I was young, maybe it has to do with all the memories I have from childhood that most people probally don't have that keeps me from disrespecting the kids/babies I don't know, but I know that if I do my part to be nice to a small person, there's a good chance they'll never forget it, we are who they look up to. And I'm the furthest thing from somebody who seems like they like kids if'n you knew me:).......
Back to that topic, tough. You should keep the kid part of the day and give your dad/parents a chance to spend time not doing some kind of baby shuffle. For one, some of the people at Six Flags parks waste enough time linejumping, playing with basketballs, and playing with big silly prizes in line and stashing them to the sides, et cetra, that they may not mind there, at least that's my own experiences(SFoG Mother's Day, no lines, all walk-ons, and we encountered so much unneccesary waiting on persons, man I'm suprised I didn't pull out my hair!). For one thing, it may not be the safest place for the little sister. It can get hot in quee, noisy, who knows. Try taking turns, it's cool.