i had a similar fantasy for the longest time about my old roommates girlfriend. we share a single room in the barrack and she would frequently spend the night. they were not passionate whatsoever and she was georgous. i felt she deserved way better and much more attention. but she was my roommate and still very good friends girl, so i made sure to keep my eyes in proper places and be very polite but never overly friendly. i kept it clean the whole time. since then they have broken up, he has moved out, and she has gotten married. im still very good friends with them both and he still has no clue i ever felt this way. she and i on the other hand had a drunken discussion on it one night maybe 2 months after their breakup. nothin happened tho, and i think its better that way. anyways moral of the story is havin these attractions and urges.... totally natural. acting on them in any way whatsoever, including the small hints, inappropriately long stares, anything that might show the interest to your girls sister. its just the wrong thing to do.