Factal

A sims game maybe. Or if you have enough power to emulate wii or gamecube you could get some nintendo games.

Rayman Origins

Pedobear - Little Girls Edition :D

COD

Postal 3

Amnesia: The Dark Descent

Lego anything (Batman, Harry Potter, Star Wars), but you might want to get a gamepad.

I would say Nimbus. It's a cool game for a kid. Well, i liked it.

My sister likes to play Portal 2

Terraria or MineCraft

killing floor :)

Crayon Phisics Deluxe

zombie shooter 2, you never know when it happens

Serious Sam

Lucid, Toki Tori, Faerie Solitaire, Crayon Physics Deluxe, Your Doodles Are Bugged! and, if she's bad ==> Left4Dead2

I would say Toki Tori !

Super Meat boy!!! ^ ^

im a wiw giwl and i luv hl2dm :D

Depends on how "little" she is.
Since steam is actually for 13+ people, hehe.

Terraria, Minecraft, Portal/Portal 2, or Torchlight (:

I want to thank your sister for Skyrim
She said that she dont like it

How hasn't this been recommended?!