My personal experince:
Run things by the parents first, Example, instead of buying that game they will never be allowed to play anyway, talk to the parents (you are probably a sibling to one of them, eh? talk to them anyway) you can instead find out nice little things like things they want but that the parents either can not, or will not get for them, but would not mind you getting for them. Think of it like this, what better tool to have as a parent than be able to not over endulge your child with something, but allow their uncle to get it for them. so next time the next big game is out " I dunno...what does Uncle Brian say about that, he seems to know more about such things..." Of course it helps to have the extra tria to drop on such things. it then becomes easier for them to "take it away for a bit' if they did not just give it to them. "I do not know, I know Uncle Brian thinks you are ready for that pellet gun, but you need to really show responsiblity.... Tell you what, I will show you how to use it, and if you keep your grades up, keep your room clean, and never ever point it at anything you do not intend to shoot, including animals, I will consider letting you use it unsupervised... now lets go out and I will show you how to use it safely" Sounds better if you had not just bought it for them for their birthday.. but from an uncle... sure. better to let the uncle (or aunt) be the over-indulgent one.
Also, you want to be there for fun stuff. When they visit, offer to take them fishing/looking for snakes/to a local event, etc. Parents would love some alone time IF they can trust you will bring them back in one piece. Again, you can do things the parents do not have time for. You take them fishing, for example, and they can bother you about fishing again. Their dad takes them fishing and he will be asked to go every weekend. If parents doing it every weekend it is not a problem, but if they do not like it... can be a real sticky wicket.
Hope that helps.