I did a fair bit of babysitting for family friends when I was 14 - 18 (I'm male).
I always worried a little bit about how to properly deal with them when things went a bit pair shaped. I babysat one family with 2 boys, and another with 2 girls. The girls would constantly fight and while I was constantly trying hard for them to like me, I would sometimes have to be quite forceful with them to stop them pulling each other's hair out! They were nice girls though and we struck quite a friendship over the years, I even began to look forward to babysitting them.
The boys? They were like angels! 12 and 9, they were both incredibly cute and well-mannered. The 9 year-old was an obviously quiet boy and he still liked to be read a story in bed, which I was only too happy to do (it really was thoroughly enjoyable). And then I'd sit with the 12 year old watching TV, talking about all-sorts. I think he enjoyed it because he saw it as having a grown-up conversation. It was a real privilege looking after them.
So you can see I babysat two quite different sibling groups, one kinda rowdy, the others not so! There was a definite reason why though:
The boys' parents, although they weren't strict, before leaving were very clear to the boys about how to conduct themselves when I was around. I got the distinct impression that they had had a few words of guidance before I arrived also.
The girls' parents, although very good parents, didn't do this with the girls and initially they saw it as "a free house without parents" which of course always ended disastrously!
So I would strongly advise some clear ground rules being set and all that. Also, the 16 year-old (who does sound like a good lad) might want to bring school work round to do, especially as he nears exams or whatever; so maybe make sure there's a workspace available? Not an absolute necessity of course.
All in all, I loved babysitting when I was a boy, thoroughly enjoyable experience