Amnesia: The Dark Descent would be a great one for her.

For little girls? Any CoD OH SNAP!
Puzzle Agent 1 and 2 (if creepy little gnomes don't creep her out :/ )
Braid, Machinarium?

what about BEEP, is totally inoffensive...
or Portal, leaving behind the murder and slavery ideas is completlly harmless
also SCP 087-B

skyrim :P

World of Goo for the win.

oh and...buy her monkey island series. That games are my childhood.

How old? Here are some suggestions which might be appropriate depending on her age/personality.

RollerCoaster Tycoon 3!!!
SpaceChem,
Worms Reloaded,
Audiosurf,
Bit.trip runner,
Botanicula,
Braid,
Cogs,
Lucas Arts Adventure game bundle!!!
Machinarium,
Monkey Island 1 & 2 Special Editions,
Peggle Deluxe,
Psychonauts,
Windosill,

I don't think this is the best place to ask suggestions for a little girl...

But here we go (Steam only, mostly puzzles or music games):

• Space Channel 5: Part 2
• Toki Tori
• QUBE if she is able to beat Portal
• Ziro
• World of Goo
• Costume Quest
• Lucid
• Chime
• Lume
• Who's that Flying

HIB games with an exception of The Binding of Issac lol.

Trackmania

TF2. It has dressup, and most players are little kids already.

Tropico 4 It's even on sale! Or just try the free play time of it.

Superbrothers No idea what it is, but it should let her know how Super you are.

Puzzle Pirates An entirely wholesome tale of pirating and puzzles. Some wenches may be involved.

Farm Frenzy Or maybe one of its sequels.

Fractal

Auditorium

Anything Nancy Drew.

Hello kitty online adventure and dora le explorateur. oscar worthy classics for toddlers.

Scary Girl, Super Meat Boy, stuff like that.

But obviously take blood off in SMB if possible

I let mine play GTA 3 on Ipad; Lost saga (mmo); guess the genre she likes would be ok. :)

Nancy Drew series?
They're pretty fun, and are a decent 5 - 10 hours or so.

I introduced one of my younger female cousins to gaming with Doom II, it may be a great title to get your sister started if you set it to "I'm Too Young To Die" and/or hand her the cheats list. However, if she turns out not to be a huge fan of turning zombies and demons to shreds with big guns like my cousin did, give her any of the latest Lego games (Star Wars, Indiana Jones, Batman, Harry Potter), they're absolutely awesome.

Most of us us This and its gonna make history in steam.

recettear

dead space 2:D

Scary Girl
http://store.steampowered.com/app/202370/

Amnesia: The Dark Descent

I'd say go with Sonic CD. Retro, and I still think it was the best Sonic game made.

Crazy Machines.

What about these following games :

Trine,
Magicka,
Beep,
Faerie Solitaire,
Dustforce,
Super Meat Boy, (If you love her, dont buy it!^^)
Titan Attacks.

Plants versus Zombies. My niece loves it.