I looooooved New Moon. It's such an empowering magazine for young girls! But pp is right...is there a craft or hobby she enjoys? If you go to a large bookstore and browse the magazine section you might find something that jumps out at you as being the right fit.
But don't limit yourself to just the "chatty" magazines...I had a subscription to National Geographic when I was around 11-12, and a subscription to Analog (a science fiction magazine) at about the same age. So if your niece has a specific passion for a subject there's almost certainly a magazine that addresses it. And if it really is a passion, she'll probably enjoy the publication even if it isn't "aimed" at the younger reader.