What do you do if you are uncontrollably attracted to your girlfriend's sister?

What if her sister is more attractive, has a better personality, nicer, more understanding and even smells better? How do I go about seeing if she feels the same way and eventually get with her without raising suspicion from the gf? I know that she will forgive me eventually for leaving her for her sister but the problem is that she might try to sabotage me now when I am still trying get with her sister in the present time.