I posted a long reply in the anime for 12-15 year-old
thread nearby this one that includes many shows I'd suggest here as well. I'll reiterate the point I made there -- unless you're showing her something that's clearly targeted toward younger or family viewers like Miyazaki's
Spirted Away,
Dennou Coil (see below), or
Chi's Sweet Home, pre-screening is a must. I'd include a parent in the pre-screening as well.
The show that I most want to recommend, but that poses difficult problems because of its dramatic content, is
Kemono no Souja Erin. You
and a parent should watch this at least
through episode thirteen before deciding if your sister can handle the events it portrays. If you want more details, and don't mind being spoiled, read the
Erin thread.
Seconds for
Angelic Layer (it was next on my list), which for US-Canadian viewers is currently free to watch at
Crunchyroll. I'd also recommend the two seasons of
Chi's Sweet Home. The first season is
fansubbed; the second season is being shown on
Crunchyroll. The currently-airing
Higepiyo, about a bearded chick (
), is a bit more edgy than
Chi, but probably still in the ballpark of acceptability.
A couple of other suggestions include
Windy Tales and the licensed
Tweeny Witches, which i described in the other thread.
Windy Tales is a very overlooked show by Production I.G about middle school girls who discover how to manipulate the wind. Some very cute cats have the same ability in this show.
A more dramatic option might be
Asatte no Houkou. One of the protagonists is your sister's age. I'd recommend prescreening this one as well however. The young girl has a few rather frightening experiences in the latter half of the show.
Finally, if your sister likes fantasy, and your parents are okay with the violence, she might enjoy
Guin Saga. This is based on a famous series of light novels that date back to 1979 about a strange man with a leopard head who comes to the aid of a young prince and princess. Guin has a number of one-of-one battles in the early going, and some of them are pretty violent. When she's a bit older, it'll be time to watch
Mononoke Hime,
Junni Kokuki,
Seirei no Moribito, and
Noein. She'd probably find them a bit boring at this age.
Edit:I can't believe I forgot to recommend
Dennou Coil. All the principal characters in this show are your sister's age, yet the story should appeal to you and maybe to your parents as well. This is an even better choice than
Erin. That girl with the pigtails third from the right in my signature appears in
Dennou Coil. Click on her picture to learn more about this wonderful show. (It's a Wikipedia link, though, so tread carefully to avoid spoilers.)