It really depends on the game.
For something like TF2, you'd probably need to run it through Steam, which won't really install on anything other than a hard drive (pretty sure the installer actually stops you from doing this). Apply this to any Steam game. I don't know how legal it is to circumvent this, so I won't address that. If your sister's computer has Steam, you're set.
Blockland should run fine off the flash drive if you use the -profilePath launch option.
Minecraft could work if you use some method of forcing it to store on the flash drive.