H.Q.
I'm an oldest - have a brother 3 years younger and a sister 9 years younger.
I started watching my brother when I was maybe 8. Not for long periods of time, more like "Nuts, I've got to go across the street for a minute - watch your brother". And as a teen was asked to watch them also.
I see nothing wrong with your teen watching her siblings. And, honestly, I'm not sure incentive is an issue. I was told this is what families do - watch out for and care for each other. I'm not saying she should do this every night, but if there are times you and hubby want to go to supper, there is no reason your teen shouldn't watch her sibs.
Make sure you lay out very specific rules and that you stress them with ALL the kids before you go. For example make sure everyone knows what time is bedtime. While you're home, everyone may know that 8:30 is bedtime, but you're gone and this is just big sis and #2 doesn't really want to go to bed. Set out specific things like that - makes things easier all the way around.