This is a question that I can only honestly answer by saying, I love both of them. Which is true. I do love her sister, even apart from romantic attraction. Sorry if that answer sounds weasly, but it is the truth.
I am completely aware that this makes me a bad person. You don't need to hit me over the head with that. It really isn't anything my girlfriend lacks in that is the concern... just how amazing her sister is. I don't feel right comparing the two, but I guess that is what we are doing. I really hate this situation and I wish the feelings would die. I've tried. They won't.