I agree w/ the touching comments.. if something like that is going to happen its going to be going on somewhere other than vacation and it's going to be a bigger issue. AND and if your going with that mentality... girls can also touch girls and boys can also touch boys. it's not just a boy/girl thing.and it's also not just a kid on kid thing... lots of adults are in on it as well. I agree w/ you have to be careful as you can't undo whats been done, but SERIOUSLY??? REALLY??? you are in the exact same room like 2 feet from each other... if something "happens" then, goodness only knows what is happend at home where there's whole bedrooms to sneak around in. I'm not saying these bad things don't happen and you don't need to be aware. However, assuming these things are NOT happening, there is nothing wrong w/ Siblings sharing a bed.
Your in the same room. and most likly going to be so tired everyone just passes out.. if it works for you, let them share until they no longer want to. But I'd ask BEFORE the trip so you can maybe bring an airmattress or something for the one who does not want to share the bed. I only have sons, but grew up in a big family ( I'm 1 of 8 3 boys 5 girls) and throughout my life and many vacations have shared beds/tents/rooms w/ opposite sex siblings and or cousins and none of us ever had a touching problem (I am very thankful!)
Now, DS6 will sleep w/ Dad, and DS8 will sleep w/ me on our upcoming vacation, simply because DH & I can not share a double. We have a King at home and sometimes that feels too small!