Hey ISO Darthshoe, your sister's art is fantastic for her age. I believe that drawing at a young age is largely responsible for becoming masterful in the later years. Continue to encourage her to draw. She will like it more and more with the feedback she receives, and as a result, she'll continually be inspired to get better and better.
If she continues at this rate, her work will be breathtaking by the time she's 16+. I've been drawing since I was little too, and people always commented that I was ahead of my time-- I wish I drew more though. I'm trying to take art more seriously now.
Anyway, tell her to keep it up and I'm really impressed.
I know she's young, but encourage her to toy with shading. I didn't really start shading until years later, but it couldn't hurt to experiment for fun.
Another thing I'd add, to compliment her skill, you should consider getting her some art books. When I was younger I loved the art books. Considering she's young, it might be a good start to grab random how-to-draw books that focus different things, like cartoons, people, anime, whatever. As for more advanced books as she grows up, I highly recommend Jack Hamm's Drawing the Head and Figure, anything by Loomis, etc. Getting a foundation as young as you can for this kind of stuff catapults your skill 10 fold-- namely if you're already into art to begin with.