Happy to see you are dealing with this with both your mind and heart. Yes, it difficult to see your brother as opponent because he no want harm to you. Don't worry, he's not your enemy, no need to force yourself. However, you can see that he sometimes not think about his actions, and in the heat of the moment, you two get too close. This to be avoided because it too stressing on you. >>93 is too harsh judging your brother, but correct in one thing: Even though there no malicious intent, he trying his luck. It great he respect your demand, but it not enough for you to stop being cautious, both towards him and towards yourself.
Frigid Onanoko try to illustrate with silly metaphore. You live in your house, the house is a zoo, your brother a baby elephant (cute!) and you the keeper of the elephant (jumbo). Happy Jumbo, upon seeing you may rush to greet you when he sees you. He likes his familiar caretaker so much and just want to be close! However, if you don't step aside, or stop the kiddy elephant in its tracks, it gonna trample you. Oops, accident! That never the intent, but then, can you trust a baby elephant to understand all that? Jumbo need to learn to keep distance and you need to teach.
Method is simple conditioning. It take time. Just keep at bay. You already showed you can do it during first two weeks since original confession, no? FO confident you can manage, especially after seeing you can and want to think through this problem. You could be a bit more critical perhaps, (even about FO's babbling nyah!) So don't worry about the 'opponent' mentality. When Jumbo is too eager, you can just shove him aside gently but decidedly. Your brother will understand that. He will respect it. In fact he probably expect it anyway.
BTW, you not so alone in your adventures. Wikipedia mention that in USA, 10-15% of college students have had sexual experiences with siblings Floyd Martinson study (which usually remains limited to kissing - definition of sexual experiences very very broad.) And then Frigid Onanoko also... well, very glad this is anonymous board! Anyway, it by no means so very odd.
Stay alert and succes to you!
~Frigid Onanoko