Well, judging from the advice so far, I suppose I'm not being all that over-protective.... I just don't want to get into the position of allowing the girls too much freedom and then having to pull them back into the idea of "family vacation."
Ed's already worried that the girls will want to spend all their time together and wind up neglecting Billy. I realize that they will want to spend some time alone... after all, that's why we're inviting Catie in the first place... to be another companion for Tricia. I'm thinking that they will take turns sitting him on some rides and on others they will want to sit together... and that's okay.
carissanboys and
Hedy: you both suggested some sort of compromise or soft test... I guess I don't mind if they go off on one ride together and then do some stuff with the rest of us. I just don't want them thinking they can branch off from the rest of the family for big chunks of time.
I could probably let them go off together when we go to the pool... Billy doesn't really like the deep end, anyway! And the lifeguards give some presence of adult supervision without them having to be "with" me and Ed.
mrsksomeday and
kimmylaj: Yes, Tricia can be mature at times (as can her cousin), but you know how this age is... they have one foot in childhood and one foot in teenhood... and they keep lifting up one foot or the other throughout the day!! And, I'm not so worried about them as I am about all the crazies out there. Not to mention all the teenaged boys....

I may just have to use baby steps...
mousescrapper and
mrsksomeday: Yes, I'm hoping we can get everyone (adults) on the same page. Right now, we'll just be talking about things casually, but when we get closer to the trip I'd like to have a "meeting" with all involved parties to set down the rules.
I must admit, it'll be easier to do this with family than with a friend of Tricia's and her parents.
I'm also still keeping my fingers crossed that DSis and DBIL will be able to come for part of the week!
maroo: 
If only Aimee

could go with the two of them, I would feel so much safer!!
Kathy