Hahahaha I know! but it's a REAL game!

I was going to suggest Dungeon Lords, but that might be a few to many steps backwards from what she is used to :P

Not really. I love Divinity II, absolutely adored it, but it's absolutely not open world, not even close. The areas you go through are fairly big and roam-y, but the progress itself is very linear - area to next area - much more so than Divine Divinity. On the plus side, Divinity II is "finishable" and very story-driven, with lots of different ways to solve quests - without any black and white moral system - and Larian's wicked and wonderfully silly sense of humour all over the place.

As a recommendation for OP's sister, I second a previous poster's suggestion of Kingdoms of Amalur (aka Reckoning, but no-one seems to call it by that name :P). In terms of gameplay, it's like a cross between WOW and Skyrim, with a better fighting system (much better, in fact). It also features absolutely gorgeous artistic design (courtesy of Todd McFarlane) and colourful, slightly cartoony graphics (one might call them WOW-esque, in fact). My complaint is that it starts out rather blandly despite the interesting premise, and I've had trouble getting into the game. But the combat system is quite enjoyable, and the artistic direction is enough to draw you in. Plus, it's fairly cheap nowadays, even new, so give it a shot. :)

Arx Fatalis, maybe? The gameworld is on a much smaller scale, but it plays fairly similarly to Morrowind and what I've seen of Oblivion.

Unfortunately, they aren't DRM-free (and therefore not on GOG), but she would probably enjoy Borderlands and Borderlands 2. If she can handle the themes in Oblivion, Skyrim, and Fallout 3, the Borderlands series won't traumatize her.

It might take a bit of convincing, but she would probably also like the Baldur's Gate/Icewind Dale games once she gets used to isometric view and turn-based combat and having a party instead of a single character. If having a party and turn-based combat are too much of a turn-off, she might enjoy Torchlight (1 or 2) instead.