they may be high they may not be, the op just doesnt know, and i wouldnt cancel a holiday becauase my child 'might' be incubating chickenpox, when ds1 got chickenpox i thought ds2 would get it as well, he was exposed to it, they were at home together the whole time but he didnt get it! he did get it a year or so later, by which time ihad ds3 and ds4, neither ds3 or ds4 caught it from ds2! despite the fact they were exposed to it. ds3 got it some time later, ds4 didnt catch it from him either and despite him and dd (no 5) being exposed to it loads, esp ds4 at pre-school (he is about the only one at pre-school NOT to have caught it!) they still havent got it.
so when my children had chickenpox we stayed indoors, however once the spots were scabbed over should i have then stayed in until i was sure the others didnt have it? thats insane, you cant live your life that way, with 5 children i would have had weeks quaranteening myself nad them, yet NONE of them actually caught it from their siblings and i dont think that is that unusual actually, esp when the younger siblings are bfed (as mine were) none of mine actually caught chickenpox until after they stopped bfeeding, i had it very badly as a child so i suspect they were getting my immunity via bmilk and that protected them? but seriously should i have stayed in with them each time waiting for them to get it? i would have had 2 or 3 wks with each one where i couldnt go out or go on holiday etc, waiting for them to maybe get chickenpox and it would have been for nothing as they didnt catch it from their siblings.
the younger sibling may get it, she may not and you cant keep a child in because they 'may' be harbouring chickenpox! op if i were you i would keep an eye on your younger dd, wiht mine the week before the spots came out they all had colds, bit of a temp etc from other friends experiences i think this is quite common?
but if i were you i wouldnt cancel a holiday because she may get it? if you stay at home and she doesnt get it what a waste of your holiday time!