Maybe it's wrong, but I have a crush on my girlfriend's sister.
That could cause some problems if you're not careful. How serious is this crush?
Well, I don't love her; she actually drives me crazy sometimes. I just have a small crush.
It is not okay to cheat on your girlfriend. It is okay to have a crush -- but to acknowledge it is a crush only, and to not act on it.
I think it would be okay to flirt with her a little bit to get her attention, but I wouldn't cheat.
That's playing with fire. If you value your relationship with your girlfriend, you need to keep your distance from her sister.