re: Alistair+++ would be free.

My sister loves Don't Starve. I'm not sure if that game has multiplayer though.

Beware that Magicka is surprisingly demanding.

Age of Wonders, Heroes of Might & Magic 2-5, Civ 4, Torchlight 2, Baldur's Gate (2 at least), Diablo 2, Hidden & Dangerous (really challenging though), Left 4 Dead 2, Dungeon Siege 3 (also surprisingly demanding), Payday: The Heist. That should give you something to play. Not all of these are designed with co-op in mind, but can be played cooperatively to some degree.

QFT - except Diablo 2 is waaaaaay too old, technologically and conceptually. Trine and Torchlight (all installments) are excellent modern alternatives, though.

You should check how many people still play it, you'll probably have a surprise :P

Whatever you do don't get her started with Bejeweled-like games, I've seen women get addicted to those types of games.
Also Farmville, World of Warcraft and once or twice Minecraft.

What? How was the Diablo 2 concept "superseded"? I have seen more step-backs recently (most infamous, D3)

Trine and Torchlight are nice but both have a pretty different (not necessarily better) game mechanic and atmosphere. Talking about recent alternatives/successors, I found Path of Exile a worthy and free alternative to Diablo 2 (and also multiplayer).

Enough with getting your sister into pc gaming, how to we go about getting into your sister?

Cinders
It's a Wipe
Gamebook Adventures 1: An Assassin in Orlandes
Long Live The Queen

Besides, if it's about the system requirements, "indie" is no guarantee that the games will be compatible to low end rigs. A lot of them have higher system requirements than you'd think just by looking at the screenshots. Of course, it also depends on how "old" that laptop is, but if it doesn't have a decent graphic card, I'd be very surprised if games like Trine 1 & 2 or Torchlight would run smoothly on it (not to speak of all the 2D retro indies using modern 3rd party 3D engines and adding Bloom effects and Shader 3.0 to the mix).

Do you read the forum and subsequently post through Google Translate? If not, why in blazes are you misquoting the living daylights out of my post? "Concept"? "Superseded"?

Diablo and Diablo 2 aged, and not too gracefully at that. The hack-and-slash genre is alive and well, but a whole bunch of technical limitations and questionable design decisions make those games a worse option given modern alternatives.

(Plus, the franchise itself, apparently, isn't something that one would consciously choose to be a fan of - as in: Diablo 3 is rather widely claimed to suck all sorts of ass, so much that recommending it to people is risky business - while Trine and Torchlight are still going strong.)

To quote you directly: "is waaaaaay too old [...] conceptually" is hard to misinterprete, even through google translate ;c)

PS: what innovations had you identified in recent hack&slash iterations? I'm highly interested in hearing about new approaches introducing interesting game mechanic ideas ... as I, it seems, missed all of them :)

Amusing enough, no-one enquires about her age.

"Into", methaphorically or literally? (O _ O! ) Also, "Hard Reset" is a very good old school FPS, "Wimp: Who stole my pants" seems like a good platformer too, "Cave Story+" is a very good platformer, "Bunch of Heroes", quite funny and enjoyable co-op, "Dungeon Defenders" seems to have an entertaining co-op too and lastly, "Vessel" a very good puzzle-solving platformer.
Other than that the rest have already recommended most of the others.

Predictable yet elegant. I chuckled.

I highly recommend Terraria! That game is so awesome I played it until I couldn't stand it. That usually never happens to be so it gets a special kind of reward for that.