Siblings do not, and should not, have to have everything be equal. Your 6 year old should not be supported in feeling "devastated" because she is not allowed to do everything the 12 year old is allowed to do. Put her in camp for 6 year olds while he's gone, or something. You could easily come up with something special for your daughter to do when her brother is gone.
Is she going to be devastated when she is 10 and she can't drive, like her 16 year old sibling can? Etc.
ETA - Maybe that's an extreme example, but I didn't want to take the time to come up with the myriad of other things she is not going to be able to do that he will. Even though your went when he was 5 -- THINGS CHANGE -- and now your sister doesn't want a little one along. I guess she's more tired now that she used to be. And your daughter wasn't even born at that time, so she doesn't know what she missed (unless you tell her). Whatever - you have your mind made up.
The only thing I would want to know, if I were you, is if your sister plans on having your daughter visit when she is 12, as her brother did. In that case, you can present the visit to your daughter as kind of a rite of passage, and something special to look forward to when she is 12. If your sister tells you she doesn't ever want your daughter to visit (which probably isn't the case), then ask her why.
However, apparently your sister sees this as an "age thing," which means that she isn't interested in babysitting 6 year olds, but prefers older kids. Which is her absolute right.
I completely disagree with you on this. It's highly unfair to your son that you didn't allow your son to go this year, just because his sister couldn't go.
Your sister is completely reasonable and within her rights to prefer older kids and not want a younger child. And it's unhealthy for your daughter to feel entitled to do everything her much older brother gets to do.