The stack is set up in a way that there are some (1-2) activities per day that last about 1 hour for specific ages (13-14 & 15-17).
Some activities are together. But mostly the kids go to meet friends and hang out, and that is not always in the Stack.
DS was 13 the last time and he had a great time. He is looking forward to our cruise in 6-2006 because of the Stack.
I have heard that one of the older (15-17) games is a bit on the mature side. But the younger kids are not suppose to attend. But you know kids, some will attend that shouldn't.
I would tell her to use her best judgement if she is uncomfortable at an activity. Most of the activites are games.
If she will be 14, I assume she will be in high school and probably have been around the older kids.
Dh and I teach from gr. k-12. We see alot of kids. Generally kids will gravitate to those like themselves no matter what the age. For example, I was very worried about a freshman girl who is very "young". She clicked immediately with a senior who was also very "young". I'm using the term "young" because it sounds better.
If she doesn't like the Stack, there of course is nothing wrong with family time. I'm sure she will want to spend sometime with her sister as well.