In our situation, it would depend on which kids were in charge, which kids were staying home, how far away we were going, how comfortable and likely the kids are reaching out to outside help, and which night of the week it is. On a weeknight where they have an established routine, it's easier to leave them alone. I'd be much more comfortable leaving DS1 and DD alone but throw DS3 in the mix and all bets are off. DS3 won't listen to DS1 or his twin sister. But he will listen to DS2. However, DS2 is at an age when I don't trust some of his decisions and he wouldn't listen to his older brother so that changes things too.

OP, in your situation, I think 6 hours is a long distance to be from your kids in the event of an emergency. It helps that you have family and friends close by who could maybe check in on your girls. But the biggest thing that would give me pause is that your daughters don't get along well. So, if DD1 decides that they shouldn't do something, would DD2 override her? Pout and ride her bike to a friends' house without clearing it with her older sister? Lock herself in the bathroom with her cell phone for 5 hours and sext the boy she's crushing on at school? Is DD1 mature enough to know not to go hang out with friends and leave DD2 alone for hours at a time?

I'm more of a worry-wort so probably in your situation, I'd have the girls stay with their grandparents or at their friends' houses for the night.