I cheated on my girlfriend. Can you blame me though?
I'm a little shocked to hear this, why would you do that?
I was fantasizing about her sister and began to be attracted to her.
It was not a good decision to do such things, and I hope you feel a little remorse because it's not right to cheat on your significant other. Relationships between you all can potentially be harmed because of this.
I don't care about that. I'm getting turned on by the thought of making love to her sister.
This is not good, if this goes much further than just fantasizing you will be getting yourself and everyone else involved into a big mess. It is best to just leave it as a fantasy if you can or get rid of it altogether by just making your girlfriend happy. I suggest the latter.