My niece and nephew are adults now and don't come spend the night or weekend with me anymore but when they were younger ... my house, my rules. The fun of coming to my house for them was that the rules were different...get to stay up later than normal, sleep in later, no chores and whatever they want to eat is what we have and whatever movie they want to watch. But arguing, fighting, hitting, not sharing...that sort of thing wasn't tolerated. Their Mom and Dad never questioned it and I did follow their rules for quite a few things if needed. My house was playtime and all about them so there wasn't a lot of discipline needed...just a time out every now and then. I would never have spanked them. Generally, it just took one time to follow thru after a warning and the behavior was corrected...they would not stop fighting, they got a warning, continued the fighting and I took them home. It was a very quiet car ride. After that, if I gave a warning...the behavior stopped.
On the flip side of that, if I was at their parents' house and they were misbehaving, I stayed out of it. I only disciplined when I was alone and responsible for them.
It's all about trust. My brother and sister-in-law trusted me and I honored that to the best of my ability.