Is there any way to play/install Portal 2 without Steam?

I bought a retail copy of Portal 2 last Wednesday and installed it onto my computer. The game is registered on my Steam account since I had to use Steam to install it. My sister wants to play the game too. I refuse to let her on my computer for various reasons. I also don't want her to have access to my account. I can only afford the one copy of the game and my sister, being the cheap twit she is, won't pay for anything on her own. If I just install it through my Steam, it will be on her computer but it won't work unless my account is the one logged in while she plays. I'm not so concerned about installing it without Steam as I am being able to play it without Steam. Is there any way that I can install it or at least have her able to play the game without needing to access my account?