I crossed the line of friendship with my best friend and now I think I'm falling for him what should I do?

Me and this guy have been best friends literally since we got to college. We both lived in the same dorm and he is in the fraternity that I frequent. He is like a brother to me and I a sister to him. We were always there for each other and he's seen me at my worst. I mean geez we ate dinner together every day at 4:30 never missed it unless you had really good reason. Well a few weeks ago we drunkenly fooled around and crossed that line. I asked him then why this was happening now and he responded "what if I told you I've been wanting this to happen for a while." Keep in mind we were drunk. Oh us getting together has been brought up before but we both agreed we would rather be friends because we didn't want to mess up our friendship. So the weekend before he asked me to come over because he wanted to see me so I did we fooled around again and stuff and we just acted the way we usually did. Anyways this past weekend I went to a party over there and I didn't notice this but my friends did. He was blatantly flirting with one of my friends and they said he wouldn't really do anything until he caught my attention then he would do something outrageous I guess to make me jealous. Well now I'm afraid we've completely ruined our friendship and I'm starting to develop feelings for him, I mean I've always cared but not like this. I really need some advice on what do. Thanks
I crossed the line of friendship with my best friend and now I think I'm falling for him what should I do?
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