i don't know, I think this sounds like you choosing between something you WANT to do and something you know you SHOULD be doing.
While I get your point about how her SD could get into trouble, I don't see this as a "want vs needs" thing. Sure, it's just "one play" - but her child is excited and as it sounds like her DH won't be there. It sounds like if she leaves, her son has no parent there. Why does he get the shaft because the SD is lying to everyone around her?
Too much of this seems to be falling on her, and that's not right. There are 4 parental figures involved here. Why is she, alone, juggling THREE kids?
I was a kid once with step-parents. When I was really young (under 6) I would try the "I want to go to mom's" card. My sister tried it throughout high school.
You know what the answer was? NO!
You all need to sit down and talk this through together.
When she's supposed to (scheduled to be or everyone agrees upon when) be at your house, that's where she stays. She cannot go back to her mother's because she's being a teenager and is pissed with the rules. When she's at her mother's she can't come running to your house because she's pissed with the rules. Tough shit for her. She's playing you all and now you and her step-dad are in a really shitty situation with it.
I'm getting incredibly stressed out reading your post. Aggh. SD issue aside, I think you need someone to help you out with the driving in the afternoons - like a grad student, someone responsible with a good driving record. It's really hard to be at back to back places (well maybe it is when the kids are older) but agg.