First of, Frigid Onanoko think your brother is stupid enough to actually desire you for such a long time. He really should (have) move on. Well, at least he talk about it, so you know. With that, you now also responsible for dealing with situation: since you are now aware that your brother likes you very much, you are now a player. You must play the game because the matter concerns both you and your brother (so two reasons to play.) That should be obvious.
Which direction should you aim for? Probably you realise from what FO is saying that pursuing romantic love with you brother is not an option. That's what Frigid Onanoko think anyhow. All good for liberal thought, all good for revolution against outdated social institutions, yeay for rock'n roll, let's go freedom of expression and all that... but this just one bridge too far.
So, advice is to get your brother to understand that he should get over you. What are you waiting for? Its been nearly two weeks since he started this and yet you no give him any direction either way! So frustrating for him no doubt - is that a loving sister?
Frigid Onanoko also wonder where this brother get the courage to say he in love with you. Has he ever had girlfriend? Sounds like not. He seem not to know what the hell he talking about. Since you are girl, you have tops on subject of love. Just insist that he don't know love as it should be - as it could be at its best, that you wish him good luck in finding a good girl in school or whereever. And maybe offer helping him in that a bit. It'd also be very good idea to "solemnly swear" that what is said between you will never leave the room - it'll be important to him. Make sure you do that. Btw, in the rare case that he try to assault you, hit him decisively: This part of the game is not to be played soft. In fact, the best preparation for such a talk is probably to resolve yourself to hitting him if it gets to this (FO think it may easily get very emotional so that's why) and then for the conversation try to avoid drifting into situations where you must hit him.
Ok. Frigid Onanoko ranting ranting ranting never stop now, right? Ok, gonna get to closing then. About the other boy. Come on girl! getting together with guy you don't even feel much for? don't waste your first love like that! Besides, you're letting feelings for your brother dominate here, where in fact you want to get out of this zone. This is loser strategy. Resolve matter with brother first. Then if you still don't mind "being asked out," well give him some signals that he get courage!
FO say byebye now!