I agree with what everyone said about parenting and discipline. By you standing firm on rules and consequences you are establishing boundaries and gaining respect from your daughters. Will they like it, not always but they will end up respecting you for it. As I have read many times on here childing will pit parent against parent esp. in these situations.
I am not saying that they are currently trying to pit you two against each other but kids will be kids. You still need to be the parent and not a "friend". Does it suck to have to be tough with them yes, but it is for their own good not ours.
You don't want to be the cool dad you want to be the respected dad. The one that your daughters will talk to because they trust you when you say that you won't tell anyone. Why will they trust you, because your actions over the years have shown them that you say what you mean and you honor your word.
Enjoy the trip and just have fun.
M 33 | W 34 | Kids: S4, S3 M: 5/28/05 Bomb: 8/22/09 EA: 8/1/09 | PA 12/26/09 W L: 10/21/09 M L: 11/16/09 | 12/09 to file SA W & Boys Move Out: 3/14/10