I can't see how it can be called overprotective when you know your toddler will be worked up after an unusual (for him) situation and are planning on acting accordingly rather than palming him off to someone else to deal with. I would instead call this 'reasonable' and 'parenting'.
If he's throwing a tantrum or is otherwise being unreasonable, then staying around to calm him isn't always the best option, and can be coddling or overprotective, yes. But if it's a scenario that is new to him or is reasonably stressful, e.g. trip to the dentist, first school play, first long flight, etc. then it is a very responsible thing to decide in advance that you will be there so you can help him through it emotionally, sooth him if he gets stressed or worked up, etc. so that the next time he will need less support and can gradually become more independent. Rather than having him go through a big, new, exhausting experience and then immediately passing him over to someone else just when he's at his worst emotionally.
Your sister clearly wants you to come along and is trying to find a way, but saying "let someone else deal with your cranky toddler" isn't solving it. You know your son better than her; you make the calls here. If you feel he'll be hyper, tired, or otherwise worked up, then good for you for taking steps to deal with that responsibly. If you thought he'd be fine with it, or if you'd done this before and he was used to it somewhat, then going and doing your own thing after arriving would be more understandable (e.g. taking citadelle's nap
"I'm afraid that won't be possible", "that won't work for us", and any similar phrase would do nicely here.