I once knew a girl who I always thought of as just a friend. That's not how she felt about me, but I didn't know it. I would have gone out with her if she'd just asked, and I think we would have had great times together. But by the time I figured it out, it was too late (would have been weird for a couple of reasons). I wish I had known.
If she's a close friend, talk to your sister about it first, and I think the worst thing that could happen is a "no". She might think it's strange that you're (a long time, close friend) interested in her, but she won't dislike you for it.
|