I like my brothers best friend. I mean LIKE like. What should I do?

So first the basics. For the purposes of this, this guys name will be Harry. So Harry and my brothers have been best friends for a long long long time. They used to do everything together. Then, Harry moved to Wahington D.C. Since then, I've liked this other guy. I tried to forget Harry all this time (two years), but now we are going to visit his family in D.C. Harry is three years older than me, likes the same music and games as me, but totally ignores me when I'm around. The guy I like now is only a year older than me, does the same sports as me, and is almost a genius (not to brag, but like me); aka Mr. Realistic. I get butterflies in my stomach just typing about these guys. I've been thinking about telling him I like him. The thing is, I don't know who 'him' is. Is it Harry or Mr. Realistic? And how should I act around them?