Well, personally, I tihnk YANBU.
I took many many long haul flights as a child, due to my parents working abroad, and once the plane had taken off we would all scatter to other empty seats, the destination wasn't a popular one so flights were usually only half full at most. My dad would find a whole isle free and lie down and go to sleep, while us kids would wander around and choose whatever seats we wanted.
Sometimes, my parents even did what you are suggesting
.
You have two DSs, both of ages where they are capable of looking after themselves, they will love it! Kids don't actually like being under the watchful eye of their parents all the time.
And I'm not understanding why you have to sit with them just because it is a family holiday? Family holidays don't mean doing everything together surely?
Go for it, if you really want to and your DSs are fine with it. Who cares what the MN jury say.