<human>: Can you provide a brief overview of the text?

First time post in reddit, sorry for any transgressions on reddit law!
As you can tell from the title, I'm a bit if a terrible person. I truly and completely love my GF, but I cannot help but find her older sister incredibly attractive and I feel so horrible whenever I think on these feelings.
A bit of self analysis here, but I think, in part, that this could be caused by the fact that while my GF is a loving, attractive and down to earth girl who is in no way focused on the exterior of people, her older sister is the self-aware really attractive party girl type, the type that would never give me the time of day throughout highschool. Now being with my GF for so long, I spend allot of time at her house and around her family, giving me personal contact with one of these party girls who never know I'm even alive. 
I am still very much a horny teenager, who very well may be just going through a phase, but I would just like to get the opinion of some of you out there.
<bot>: love my girlfriend %100, but feel sexually attracted to her older party girl sister, feel like a terrible person. Is it just a phase an 18yr old could be going through?