I say go for it. Keeping your feelings bottled up is going to destroy you and if she went with someone else, it'll devastate you. If she likes you too, that doesn't mean you have to rush into anything. If you truly love and respect each other, you will help each other to grow... read 1 Corinthians 13:4-8a and you will understand. BTW, nothing wrong with you... we have all experienced it.
Oh, also, my fear is twofold.... I doubt any sister would actually admit to liking me, and if I admitted feelings to someone I liked I think she would be to busy rolling around in laughter to notice me slinking away... but you go for it, lad!