In your original posting, you said yourself that John explicitly told you he had a girlfriend. If it wasn't official, he would've just said dating other people and/or not committed to anyone. He didn't mention anything about an open relationship either, and if you didn't ask about commitment or being open, you can definitely assume closed and committed.
Then, you go on later to say that he "broke up with you." Maybe he "broke up" with you because you were coming on too strong and interfering when he already had a girlfriend. The timing and the cards didn't work out. Nothing wrong with that. There are other people similar enough to "John" or different in another cool way in this world that can be for you possibly.
I do think you should take a few days breather at least, maybe a few weeks. Consider talking with a counselor about it even if you only meet them like 5 times. Afterwards, I think you should talk with John 1-1 in-person in a public place where everyone can see both of you, but if someone comes up to you, just ask them for some private time for personal conversation. Tell him your intentions of the conversation beforehand so that he understands and doesn't think you are trying to do something underhanded (such as I would like to be as honest with you as I can about my prior feelings. When is a good time? etc.) In this conversation, you should be honest about your feelings about your sister to him, and that if his situation ever changes and you are still single, that you hope he will consider you. Then, maybe consider asking for platonic friendship. If you can't deal with being only platonic, then just be cordial when you see him in-person and move on. I stress "platonic" because maybe it's more in the lgbt world then when you say "friendship" that it could imply "friends with benefits". I get the feeling he would be open to such conversation since he could be so open with you.
