To clarify my last response...
First, I think you need to be aware that the NSPCC guidance is that under-16s shouldn't be left overnight. If nothing happens, fine. If something does happen, you may get the police/social services sniffing around, rightly or wrongly.
That said, a 15 and 16 year old would probably be ok. I personally would wait till both are 16 as I wouldn't be able to relax going against the NSPCC guidelines (and worrying I might get into trouble) so I wouldn't enjoy myself.
Second, 200 miles is a long way away for a first overnight stay. I would agree with PPs suggesting a trial run first where you are closer to home. Alternatively, make sure someone trusted is much closer and 'on call' to help.
Third, it depends on where you live and how safe it is.
Fourth, even the best-behaved kids can get into trouble.
My two are younger and definitely not old enough to be left overnight yet but a similar age gap (11 and 13, albeit the 11yo is very nearly 12). I've been leaving them for a couple of hours max since they were 9 and 11 and we've never had a problem but I'm still wary of leaving them a full day in the holidays. Might try from this summer, when the 11yo is 12. At 15 and 16 I doubt I'd leave them overnight - at 16 and 17 I probably would.