I dont personaly agree that it is okay. simply because i'm not attracted to any of my sisters. however, depending on a persons personal experiences, i cant see anything that tells me why in some situations it may be okay. i dont know how common it is, i mean the way you typed it it seems like your current gf is your sister. lol. love is eternal. if 2 "souls," if you will, (if not then replace it wiht spirit, god, essence, self, conscienceness, what ever you want.) are simple made for eachother, then in one lifetime or another they may end up being related in some way. if that happens and they both know it. then i cant see the problem with that. however if its just a horny older sibling then thats a whole different story.
A sense of being part of the great all-inclusive life prompts us to reflect on our own place and on how we ought to live.