Is it normal to be attracted to my girlfriend's sister?

I am 26 and have been dating this great girl who is 23 for almost a year now. We actually met in college and we were friends for several years. But not until recently after she graduated did anything romantic happen. It was all down to luck that she got a job near where I work so we can see each other. However, a few months ago, her parents were going on a little trip and she and her younger sister were alone for the weekend and she wanted me to come over. So I did, and automatically I was more physically and emotionally attracted to my girlfriend's sister than I was to her. First of all, her sister is 17, still in high school, and has a better body and better boobs than my girlfriend! And not to mention she looks so much younger, in shape, and happy! I love her personality too, it's so innocent, sweet, naughty, and exciting! Another thing is that my girlfriend told me she plans on going to graduate school which means she will be away for a long time, and so I saw that I had a shot with her sister and we have been texting and chatting online for a while now behind my girlfriend's back.