I've admitted on this forum before, that I did everything for my D, who at the time wasn't a complete spoiled brat. I absolutely loved taking care of her and threw myself wholeheartedly into parenting her. By 4th grade I was amazed to hear that some of her friends actually woke up themselves to alarms, dressed, fixed their breakfast, packed a lunch and woke their parents up to drive them to school.

I still think that's young but at the time I considered it child abuse. I did her hair, helped with homework dressing, ect... for way too long. My next child was 9 years later, and he has had repsonsibilities throughout. He has chores (she didn't know what one was) no allowance if he doesn't fullfill his obligations without being hounded. He is capable of fixing a 4 course meal, but she never could because I did everything for her. He's 12.
I imagine your sister has figured out or will shortly that she isn't helping her. Its easy to "love a kid too much". I learned, from experience that loving a child means to let them fail on their own, so that they can learn from their mistakes. I did not have what it took to let my daughter fail and learn on her own. They don't come with handbooks. Now I will tell you that had she talked to me like that, or anyone else, she would have been visiting next week. BUT... young teens... especially girls turn into little monsters for a few years while their hormones get straightened out. Its not abnormal for them to act like spoiled brats.
I think that if an oportunity arises that you could talk to your sister without saying anything negative, that you might be able to mention that maybe your neice needs to be taught some responsibility slowly so that she will be a functional adult or able to handle things on her own at college. I had to pull back from my daughter all together about 16 maybe... you're on your own. She's capable of doing anything in the world herself, but she'ld rather me do the not so fun stuff.