hey there Tampagirl ~ I too am sorry this happened. Yes, he crossed some serious boundary here, and to me, seemed to equate your obvious affection as an invitation to be intimate.
If it were me, I would really try hard to communicate with him that you want him in your life, As A Brother. You do love this connection, right? He clearly cares for you too, but again, clearly, got it wrong about how to proceed.

I am a big fan of writing things out, and do so with anyone in my RL that I have issues with. I am far more clear and can get out what I want to without it becoming something else. Is that something you can do with your brother?

For me, honesty is best. If the air is clear then you can proceed the way you feel most comfortable. Don't be ambiguous, tell him why you feel as you do. Do you love him less for this great error in judgement on his part? Do you hope to repair the relationship and get it back on the right track?

Write and write some more, say everything you want to say, then Don't Send. Give it a week, revisit it, show it to your T or best friend or both if you have them, edit like crazy to get the feelings, good & bad, clear, then wait another week to send. Reread one more time and then if you feel ready to have a conversation, via email, do that. Tell him you don't want to close the door, (if you don't) and wait for his reply.

He will likely be embarrassed and unsure how to respond because he will now see unequivocally that he did the wrong thing. But if you make it clear you still want him in your life, (if you do) in spite of this major breach, he will come around. Perhaps it will remain private between you. That may help. I don't think he sounds like a monster or a molester or other deviant, but I do think he found you very attractive & looked at you as a woman instead of a sister that night. He may decide he can no longer have you in his life. These are things you both need to decide.

Sometimes inappropriate things happen between adults. You are at a safe distance in RL. It can be up to you to make it a bad memory and move onto a better relationship.
I wish you only the best as you work through your feelings about this.