IS IT OK TO NOT tell my BEST Friend this...even thou she knows I think..?

I feel she already knows but Iike this guy and he likes me back and weve slowly been getting to know eachother and were both acting very casual about it. I've told her I have been talking to him online didn't say anything else about it. Were all friends in college, but since I've heard college relationships can get awkward if it doesn't work etc so If I tell her she may start talking looking at him when I'm there and you know being childish about it etc I don't feel comfortable telling her but I know she knows as she has been saying hello to him and sitting closer to him so I can sit near him as she thinks I'm shy around him but really were just being very casual. & I know my best friend isn't interested as she is a lesbian. Anyway she keeps hinting for me to tell her or if his name randomly comes up anywhere she has a smug look on her like 'I know you like him but why won't u tell me'.. I just don't want to tell her as I don't see me and him going anywhere since were complete opposites and the only thing we have in common is music, style, easy going, we both make jokes, we both balance fun & work well, but the main thing why I don't want to be involved is because he drinks alot with friends and smokes pot. As I can tell if were online talking he would hardly speak but if its a college day and we talk online after class he would be talkative so he would seem sober lol.. So.. help?