Old chaperone

Tonight the little sister is having a going away party for a friend and a couple of the kids from church are spending the night. Its a co-ed shindig and I hate it. Regardless of how many responsible adults there are around I think 16/17 year olds should not be staying the night together. 
But since it hasn't been that long since I was on the other side I agreed to be an extra set of eyes. FMIL is naive enough to think that nothing will happen right in front of her face but stuff already has.
Ah young and dumb.
That's all they are.
I wish the believed me when I tell them how things will end up.