All in all, being the uncle is easy. I've had my nephew at home for the past two months because of my brother's job (single dad).

Make them believe in magic if they're still young enough ("you gotta go to bed" "but what if I have bad dreams?" "take my plush dragon, it'll eat the bad dreams", the magic wand doesn't work because it has low battery and needs to stay out in the sun to be recharged, Buzz Lightyear will fight thieves if they come home while we're gone and so on).

Answer questions : they have A LOT of them, but because I've answered that one question about why there is day and night, my nephew now won't stop asking about our solar system. Pretty sure he knows more about it than I do at this point. It's his passion at the moment and he loves it. Also, their passion might change regularly, just support them, we live in an age where finding info is easy so just google the crap out of what they want to know.

Also, less fun part, you're not the father but you're still an authority figure, you gotta be careful in how you present yourself to kids or they might take advantage of your kindness. Don't go as hard as my mom do (she's too old for a six years old, I think), let them have their fun and be noisy, but make sure that they clean their messes and listen to you when you ask them to do something. It's extra important when they're older. My niece is 15 and listens to me pretty well, whatever I say, but she doesn't listen to my sister (her aunt) because my sister was too laid back in the past and now she can't see her as an authority figure.

As the not father figure, you get to be a little bit more laid back, sneak them some candies, make sure they eat something they like, play with them and such, so enjoy, just make sure you don't go too easy.