How to act around the guy you like.?

I just recently realized that I like this guy. Our families have been friends since before we were born and have always considered each other "cousins." (Although we really aren't related at all) When we were younger we were very close and hung out all the time, but not so much anymore, now that we've sort of found where we belong. He's two years older than me and therefore I automatically ume he wants nothing to do with me because he's more mature. I am great friends with his sister and she keeps telling me that I shouldn't worry about the situation and that no one knows what will happen in the future. But, my question is, how should I act around him, not only to impress him, but so he will at least notice me in a way where he's not condradicting me or ganging up on me rudely. He's not a rude guy at all, it's just toward me he gets quieter and says few things to me, and when he does, it's always against me. Help??