How old is your niece? Girls at a certain age would be really on the stuff of princess, especially when your sister-in-law let her watch so much Disney princess cartoons. My daughter also likes princess and she wants to be a princess. When I told her that a princess will not whine and do something naughty, she then listens. At least, it is not 100% bad. Of course, if your niece thinks that she can control everyone like a picky princess, it is another question.
Totally agree. Since my son starts to go to preschool 6 months ago, he speaks to me mostly in English. I have to keep reminding him that if he does not speak in English, I am not going to answer him ...