In terms of 1st RPG and the time any RPG consumes, will she play often? If she's playing an hour a day, is that okay? Is that spending as much time, or more, than doing homework, chores, and other things?
However, other than the gore, I see nothing in this game that any teen could not handle. In fact, Tweeners could probably handle what this game has in it.
Discussion is very important. If there's something bothering you, you should talk to the appropriate persons involved. [Internet forum joke here.] Assumptions (like thinking "Eh, she'll be fine") are mean when they involve others carrying your weight when you're wrong--as I've learned.
Skyrim is fun!
For some variety, at thirteen we've gone through several coming-of-age books: The Outsiders; Maniac McGee; Hatchet; and others. All involving abandonment, divorce, survival, death--though considerably watered down, since it's up to the kids' imagination.

), but if they really aren't comfortable then it's their decision. You could try discussing it with them more though. But be honest - talk about the things which they may have a problem with and see if you can't change their mind. But if not, the niece can just revisit it later on - it's not like the game has a sell by date.