Girlfriend's 19 y/o younger sister

Hey,
I am constantly finding myself fantasizing about having s** with my girlfriend's younger sister. She is so tight and hot and really has everything my girlfriend does not. It is driving me crazy if I only had a chance of being with her, she would be the happiest girl in the world, oh my god I am so motivated to be with her it is not good I tell you.
What do I do help.