@emshae,
Well, first, how important is he as a friend? If things progress to romance -- or if you make it clear that you're interested in him and he's not interested in you -- you could lose the friendship.
Is he really your friend separately from his sister, or do you all kind of hang out in a group?
How big of an age difference is there?
(Sorry to keep answering questions with questions, but this kind of thing really depends on specifics.)