As far as I know, you can't do that. If one of you wanted to play single player, then you could possibly set one computer to Offline Mode before starting the game, and then the other person should be able to log in on the other account and play multiplayer, but both of you being online at the same time should not be possible.
Keep in mind, this is technically not allowed. Steam's agreement says you're not allowed to share your account or your games, that includes your fiancee. I don't think they actually do anything about it as long as you're not talking about it on steam chat or their forums. However, you still risk losing your account and all your games when bypassing their rules.
EDIT:
As of mid 2014, Steam has released Steam Family Sharing, which enables the sharing of Steam libraries. However, this system does not work on a per-title basis (only during the beta), but rather - If one title is being played, all other titles cannot be played (unless it's the same user that's using the library that is launching the second/whatever-is-after-first program).
Very, very simple answer if you have the time to do this, and an NTFS file system*.
*Don't know what this is? Easy: Do you have Windows Vista or 7? If yes, then yes. If not, then "maybe", and you'll have to poke around to see. Disclaimer: This works on XP systems as well, but I haven't done it for a little while. Process for XP is different.
- Keep Steam as it is. That's right, completely as it is.
- Cut and paste your sister's games from your STEAMAPPS folder (in your X:\Program Files\Steam folder) to her portable drive.
- Now open a command prompt. This can be found usually in the "Accessories" tab of your start menu, or some computers can just RUN a command called "cmd".
Enter the command "mklink", and press enter. It'll tell you that you can run a command like:
mklink /d "x:\program files\steam\steamapps\sister's game 1\" "s:\sister's game 1\"
Work out the appropriate file placement, then if it's all lined up right and you've filled in all the info as it should be according to where your Steam is installed, and where your sister's games are, press enter.
- Enjoy knowing that your sister's games are no longer taking room on your own hard drive.
Surprise Bonus Round!
Hey kids! Tired of your portable drive that you never unplug from having "performance issues"? This is because Windows tends to prevent disc write cache on any drive it's concerned may be unplugged without warning. By default, all USB drives count as this.
Want to milk better performance from it? Here's some easy steps*:
*Disclaimer: Though this process works on Windows Vista and Windows XP systems, it is slightly different to what is described below. Always use adult supervision when playing with your hardware settings!
- Make sure your drive is connected to a high-speed USB port. It's not worth doing this on an old USB-1
- Go to "My computer"
- Right-click the drive you have plugged in. Click "Properties"
- Go to "Hardware", and click on the drive you need to alter, and go to "Properties" again.
- Under "Policies", turn to "Better Performance".
This now means that Windows will start using your USB drive as a high-performance drive, like the ones inside the case. This also means that "bad things" can happen if you pull the drive out, without using the built-in Windows "Safely Remove Hardware" button, found conveniently near your clock, in your System Tray.
Thanks for watching, kids! And remember:
Spyware no spying!
Best Answer
Yes, you can share your games with your family. Steam recently introduced Family Sharing, a system where a user can share his library with the rest of the family and others who use the same computer.
You simply need to enable Family Sharing on the computer you want to use:
Note that two users still cannot play the same copy at the same time.
Also note that not all games support this system, or have only partial support. For example, you cannot participate in CS:GO's competitive matchmaking while borrowing the game from another account. Most games with 3rd party DRM, such as Uplay, will cause issues and might not even show up on the list.