just fuck each other
i appreciate the affection you have for your girl's sister. However, unless this is just recent...YOU HAVE A GIRLFRIEND, YOU DUMB SHIT!! I don't care how much you don't want to admit it...but it's the truth. And more likely...she probably still like you A LOT...yet you are not noticing it. Maybe her sister is more interesting, but you have to also realize that she's someone YOU CANNOT touch because of the relationship that sister has with her man (plus, you would probably die by the hands of your girlfriend). Pay attention to your girl...you might be missing out.
However, if this continue to affect you...then literally let the sister know how you really feel. It may hurt (and in the long run, hopefully she'll keep it quiet), but at least try. This lodging feeling in your heart will only consume you more with confusion and frustration. And quite honestly, the future may be grim for your future visitations to their home if you don't do it.