I'm sorry, well not really, but that sounds truly selfish. You are keeping your child from an experience that he would love and from which he would benefit because you can't go.
Not because you don't trust your SIL, not because he has trouble being away from you, but, basically, "If I can't have fun, you can't either."
It also sounds like a terrible way to treat your SIL. She watches your child for free and, apparently, does a great job of it. She does a LOT for you, and trust me, I have 6 kids, free always reliable child care is a HUGE gift, and you deny her this. Again, not because you don't trust her or because you are worried about your son's ability to be away from you, but because you can't go.
I understand not wanting to travel with a baby, I have a 5 month old. Not all babies are easy or travel well.
Also about the "magic." Yes, he went once, but I think your sister is thinking it would be lovely to go again and experience that again before he gets older. There isn't a reason, other than your own selfishness, that he can't go with her this year and go with you in the future, too.
I could totally get behind you if you had come up with ANY other reason other than, "we can't go, too."
...and you should pay her for child care. That is truly taking advantage.