Man, I second what billy says here "I don't want to come across as belittling your feelings, and I have known plenty of couples that have been together since preteenage. But in my experience, although I had plenty of heartbreak with "sisters", I'm now very glad that I dodged every one of those bullets!*
I've had a few girls that I liked like me back, and now in my current position I'm happy not to be stuck in that mess...
Bro, I suggest that you follow your dreams, make something of yourself and worry about the women later. As the saying goes, "there are plenty of fish in the sea".
I know hormones are strong and all, but you didn't mention whether or not this sister felt the same way for you as you do for her. This plays a huge role in everything as well. Have you told her how you feel about her? When will you? What if she doesn't like you back?
Peace and best of luck with whatever decision you make,
Garrett