Okay here's my advice:
Don't tell your sister, mom, dad, friend, or anyone about that you like her. Your sister will keep you guys apart and the others may link the information to her. Okay, if she doesn't like you like that right now, it is fine. All you have to do is wait for that oppurtunity. Let's say she comes over to your house because she's upset and needs to talk to someone. Well, murder your sister with an ax and comfort her and wambam you're both naked. On the real, just be there for her and be kind of like that caring yet VERY mature sibling (immature things will remind her of the age difference). Don't make a move first though or you could get shanked.