Being an uncle can be a lot of fun. It really depends on whether or not your sibling uses their children as pawns when they have their histrionic episodes, where they tell their kids that despite the fact you live in the same house, they are not to talk to you, and then she'll make things as unwelcoming as she can even though she's just as much a guest as you are and WHO THE HELL DOES SHE THINK SHE IS?! DOES SHE REALLY THINK SHE HAS ANY GODDAMNED SAY AS TO WHO I CAN AND CAN'T TALK TO?! WELL, SHE CAN GO TO HELL! I'VE FREAKING HAD IT! I'M SICK OF NOT BEING ABLE TO BUY STUFF FOR THE REFRIGERATOR BECAUSE SHE'S GOT ALL HER CRAP IN THERE. BUT OOOOHHHH, THAT'S FOR THE KIIIIIIDS, SO THAT MAKES IT ALL RIGHT TO MAKE LIFE COMPLETELY UNBEARABLE FOR BOTH ME AND MY PARENTS.
Anyway. So long as your sister isn't completely unhinged and isn't intent on making life as hellish as possible, being an uncle can be fun.