If they are breaking the law then you should at a minimum tell the landowner. He may have
given his nephews "permission" to hunt, but I'm sure he doesn't want them breaking the law out there.
That might get rid of the problem if he's a good landowner and limits or prohibits their hunting.
If we let someone hunt our land and they broke the law (not something like a hunter's orange violation),
like that, they would not be welcome back.

In basic nuisance situations where they are not breaking the law, blood will usually trump you, so
you probably just have to decide if it is worth it to you. I had some land to hunt closer to where
I live, but the relatives (nephews like you) with their constant four wheeler riding, shooting, and
crazy crap were too much for me.