You should NOT "just get over it and go along." This is an event YOU designed and planned and invited people to. You get to make the rules and the itinerary.
Unless your big sis is doing all the driving (which would be supremely foolish!

), what forces you to go where she wants?
She's in the car, you're driving, she says, "Can we just stop here for a second?", you say, "We can't today because this is DD's day -- sorry!" and keep driving. OR ... develop a "shopping / dining / movie itinerary" before the big day, e-mail it to all concerned, and let them know that "in order to stay on schedule and get done everything DD wants to do, there will be no deviations!" -- do it with a smiley face, or an overly-stern photo of a military general or something that indicates that you're saying it in fun, but are serious.
Has anyone seen "the list" of which your mother speaks? Do you know what's on it? Could it be that your mom has exaggerated what your sister has on this list and that it's not that big of a deal? I mean, if she knows that you guys are going to Mall X, for example, and one of her errands is IN Mall X and she can do it quickly while you and the other girls are in The Gap or whatever, that's one thing. If she's expecting you to go 20 miles out of your way to pick up her cake topper, that's something else.
I would call your big sis, tell her that mom said she had a list of errands for Saturday and you wanted to let her know how crowded the itinerary was and that there wasn't alot of wiggle room between things your DD wants to do. Ask if she wants to drive with you (noting that there will be no detours) or drive seperately, so that she can do her errands and meet up with you for dinner or the movie or whatever is last on the list. And then stick to your itinerary. If she's supposed to meet you at 3pm at the movies and she doesn't make it, then you go to the movies as planned and she joins when she can. Your sis can't ruin the day unless you let her, y'know?
But there is certainly compromise to be had here. There are a multitude of ways that sis can be part of DD's celebration without taking it over.