I am attempting to find some new games for my sister to play. Right now, the only games she will play are Oblivion, Skyrim, and Fallout 3. I've been trying to find some games like them that she might like, but I can't seem to find anything. Anyone have any suggestions? No games like the Witcher, though, as much as I love it, I don't think my sister is quite mature enough for it.

Is she a Star Wars fan?

WAIT A MINUTE...
The Fallout games have exploding children and you're worried that Witcher might be too mature? :P

Fallout New Vegas...

*resists the urge for an innuendo*

Forget to mention Fallout New Vegas.
I don't think she's ever seen Star Wars, she might like it though.

"The Fallout games have exploding children and you're worried that Witcher might be too mature?"
You'd have to know my sister. The Witcher is far too intelligent for her, deep storyline with racism, relationships, ect, she'd fall asleep.

Hmm, the Gothic games might be suitable.

Well KOTOR 1 and 2 are great RPGs that remind me a lot of the Elder Scrolls games but they tend to go over better with fans of the movies, books, lore, etc because of course it's all that music, light sabres, Jedi, Hutts, etc...

I've seen chicks who look like hutts.

Risen 1+2

Gothic 2 (the first game has unusual controls and may have problems on newer rigs, 3 didn't achieve 2's quality, and 4 is something very different done by a different team)

Here are three just off the top of my head...
Gothic 3 (completely open world, has magic, Orcs and Goblins)
Mount & Blade: Warband (completely open world albeit in a different way than The Elder Scrolls games, no magic, no elves/orcs/etc)
Two Words II (somewhat open world, has magic, has orcs)

Risen looks like something she'd like, I'll have to try it out.

Oh and give Planet Explorers a try. It's still in alpha, but it's highly playable as it is, gameplay and setting wise it's like a combination of Minecraft, Outcast and Morrowind. If you buy it from the developer's website you get a DRM free version and a Steam key.

I had been wondering if Divinity II was Elder Scroll-ish.

Ore no Imōto ga Konna ni Kawaii Wake ga Nai

A TV show all siblings should watch.

Like many others said, go for any Piranha Bytes game, especially Gothic 3 and Risen if you value big open worlds. They are a lot harder than the Elder Scrolls, but also more rewarding and (imo) better in almost every aspect.
If you want to play another Bethesda title, Morrowind immediately comes to mind! It's my favourite ES game, plus you can dramatically enhance the graphics with mods without touching the gameplay.